[release/10.0] Fix heap_segment_used watermark after compaction - #128342

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[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
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janvorli:backport-10-gc-compact-phase-used-watermark

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@janvorlijanvorli commented May 18, 2026

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Backport of #128217 to release/10.0

Customer Impact

  • Customer reported
  • Found internally

GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.

Regression

  • Yes, introduced when GC regions were enabled
  • No

Testing

CI tests, local testing using targeted repro app from the customer, GC tests

Risk

Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.

@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli added this to the 10.0.x milestone May 18, 2026
@janvorli
janvorli requested a review from VSadovMay 18, 2026 22:25
@janvorlijanvorli self-assigned this May 18, 2026
CopilotAI review requested due to automatic review settings May 18, 2026 22:25
@janvorlijanvorli added area-GC-coreclr Servicing-consider Issue for next servicing release review labels May 18, 2026
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Pull request overview

This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.

Changes:

  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).

@janvorli

janvorli commented May 18, 2026

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Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!

Edit: The stress tests passed full 4 hour runs on both of my test machines!

@JulieLeeMSFT

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Fixes #127687.

@JulieLeeMSFTJulieLeeMSFT added Servicing-approved Approved for servicing release and removed Servicing-consider Issue for next servicing release review labels May 19, 2026
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@janvorli, once it is code reviewed, we can merge.

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LGTM. Thanks!

@JulieLeeMSFT
JulieLeeMSFT merged commit da8ffe6 into dotnet:release/10.0May 20, 2026
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Guess it should be assigned the upcoming milestone 10.0.9 ?

@BenV

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Guess it should be assigned the upcoming milestone 10.0.9 ?

@janvorli is there a good way to track which 10.0.x release this will land in?

@janvorli

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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.

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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.

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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.

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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?

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var __re = new RegExp('^' + "github\\.com" + '

Skip to content
[release/10.0] Fix heap_segment_used watermark after compaction - #128342
Merged
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
May 20, 2026
Merged
[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
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@janvorli
@janvorlijanvorli
commented
May 18, 2026

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Backport of #128217 to release/10.0
Customer Impact
  •  Customer reported
  •  Found internally
GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.
Regression
  •  Yes, introduced when GC regions were enabled
  •  No
Testing
CI tests, local testing using targeted repro app from the customer, GC tests
Risk
Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.
@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli
added this to the 10.0.x milestone
May 18, 2026
@janvorli
janvorli
requested a review
from VSadovMay 18, 2026 22:25
@janvorlijanvorli
self-assigned this
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Pull request overview
This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.
Changes:
  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).
@janvorli
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cc: @BenV
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May 19, 2026•

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cc: @BenV
Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!
Edit: The stress tests passed full 4 hour runs on both of my test machines!
@JulieLeeMSFT
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May 19, 2026

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Fixes #127687.
@JulieLeeMSFTJulieLeeMSFT
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@janvorli, once it is code reviewed, we can merge.
VSadov
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LGTM. Thanks!
@JulieLeeMSFT
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merged commit da8ffe6
into
dotnet:release/10.0May 20, 2026

126 of 129 checks passed

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@BenV
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Guess it should be assigned the upcoming milestone 10.0.9 ?
@janvorli is there a good way to track which 10.0.x release this will land in?
@janvorli
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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.
@snakefoot
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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.
@janvorli
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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.
@snakefoot
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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?
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, 'i');
if (__m === '*' || __re.test(location.href)) {
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})();
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var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
[release/10.0] Fix heap_segment_used watermark after compaction - #128342
Merged
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
May 20, 2026
Merged
[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
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@janvorli
@janvorlijanvorli
commented
May 18, 2026

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Backport of #128217 to release/10.0
Customer Impact
  •  Customer reported
  •  Found internally
GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.
Regression
  •  Yes, introduced when GC regions were enabled
  •  No
Testing
CI tests, local testing using targeted repro app from the customer, GC tests
Risk
Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.
@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli
added this to the 10.0.x milestone
May 18, 2026
@janvorli
janvorli
requested a review
from VSadovMay 18, 2026 22:25
@janvorlijanvorli
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Pull request overview
This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.
Changes:
  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).
@janvorli
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Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!
Edit: The stress tests passed full 4 hour runs on both of my test machines!
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Fixes #127687.
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@janvorli, once it is code reviewed, we can merge.
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LGTM. Thanks!
@JulieLeeMSFT
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merged commit da8ffe6
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dotnet:release/10.0May 20, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@BenV
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Guess it should be assigned the upcoming milestone 10.0.9 ?
@janvorli is there a good way to track which 10.0.x release this will land in?
@janvorli
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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.
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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.
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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.
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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?
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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
[release/10.0] Fix heap_segment_used watermark after compaction - #128342
Merged
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
May 20, 2026
Merged
[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
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@janvorli
@janvorlijanvorli
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May 18, 2026

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Backport of #128217 to release/10.0
Customer Impact
  •  Customer reported
  •  Found internally
GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.
Regression
  •  Yes, introduced when GC regions were enabled
  •  No
Testing
CI tests, local testing using targeted repro app from the customer, GC tests
Risk
Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.
@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli
added this to the 10.0.x milestone
May 18, 2026
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requested a review
from VSadovMay 18, 2026 22:25
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Pull request overview
This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.
Changes:
  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).
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May 19, 2026•

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Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!
Edit: The stress tests passed full 4 hour runs on both of my test machines!
@JulieLeeMSFT
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Fixes #127687.
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@janvorli, once it is code reviewed, we can merge.
VSadov
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LGTM. Thanks!
@JulieLeeMSFT
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merged commit da8ffe6
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dotnet:release/10.0May 20, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@BenV
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Guess it should be assigned the upcoming milestone 10.0.9 ?
@janvorli is there a good way to track which 10.0.x release this will land in?
@janvorli
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May 26, 2026

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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.
@snakefoot
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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.
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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.
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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?
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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
[release/10.0] Fix heap_segment_used watermark after compaction - #128342
Merged
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
May 20, 2026
Merged
[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
Conversation
@janvorli
@janvorlijanvorli
commented
May 18, 2026

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Backport of #128217 to release/10.0
Customer Impact
  •  Customer reported
  •  Found internally
GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.
Regression
  •  Yes, introduced when GC regions were enabled
  •  No
Testing
CI tests, local testing using targeted repro app from the customer, GC tests
Risk
Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.
@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli
added this to the 10.0.x milestone
May 18, 2026
@janvorli
janvorli
requested a review
from VSadovMay 18, 2026 22:25
@janvorlijanvorli
self-assigned this
May 18, 2026
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review requested due to automatic review settings
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Pull request overview
This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.
Changes:
  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).
@janvorli
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May 18, 2026

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cc: @BenV
@build-analysisbuild-analysisBot
mentioned this pull request
May 19, 2026

Open

3 tasks
@BenV
BenV
commented
May 19, 2026•

edited

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cc: @BenV
Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!
Edit: The stress tests passed full 4 hour runs on both of my test machines!
@JulieLeeMSFT
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May 19, 2026

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Fixes #127687.
@JulieLeeMSFTJulieLeeMSFT
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and removed

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May 19, 2026
@JulieLeeMSFT
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@janvorli, once it is code reviewed, we can merge.
VSadov
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Comment threadsrc/coreclr/gc/gc.cpp
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LGTM. Thanks!
@JulieLeeMSFT
JulieLeeMSFT
merged commit da8ffe6
into
dotnet:release/10.0May 20, 2026

126 of 129 checks passed

@dotnet-maestrodotnet-maestroBot
mentioned this pull request
May 20, 2026
 Merged

@snakefoot
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May 21, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@BenV
BenV
commented
May 26, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@janvorli is there a good way to track which 10.0.x release this will land in?
@janvorli
janvorli
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May 26, 2026

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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.
@snakefoot
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May 26, 2026

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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.
@janvorli
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May 26, 2026

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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.
@snakefoot
snakefoot
commented
May 26, 2026•

edited

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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?
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@janvorli@BenV@JulieLeeMSFT@snakefoot@VSadov

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
[release/10.0] Fix heap_segment_used watermark after compaction - #128342
Merged
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
May 20, 2026
Merged
[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
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@janvorli
@janvorlijanvorli
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May 18, 2026

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Backport of #128217 to release/10.0
Customer Impact
  •  Customer reported
  •  Found internally
GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.
Regression
  •  Yes, introduced when GC regions were enabled
  •  No
Testing
CI tests, local testing using targeted repro app from the customer, GC tests
Risk
Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.
@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli
added this to the 10.0.x milestone
May 18, 2026
@janvorli
janvorli
requested a review
from VSadovMay 18, 2026 22:25
@janvorlijanvorli
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May 18, 2026
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Pull request overview
This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.
Changes:
  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).
@janvorli
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May 18, 2026

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May 19, 2026•

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cc: @BenV
Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!
Edit: The stress tests passed full 4 hour runs on both of my test machines!
@JulieLeeMSFT
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Fixes #127687.
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@janvorli, once it is code reviewed, we can merge.
VSadov
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LGTM. Thanks!
@JulieLeeMSFT
JulieLeeMSFT
merged commit da8ffe6
into
dotnet:release/10.0May 20, 2026

126 of 129 checks passed

@dotnet-maestrodotnet-maestroBot
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May 20, 2026
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Guess it should be assigned the upcoming milestone 10.0.9 ?
@BenV
BenV
commented
May 26, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@janvorli is there a good way to track which 10.0.x release this will land in?
@janvorli
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commented
May 26, 2026

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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.
@snakefoot
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May 26, 2026

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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.
@janvorli
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May 26, 2026

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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.
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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?
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, 'i');
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injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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[release/10.0] Fix heap_segment_used watermark after compaction - #128342
Merged
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
May 20, 2026
Merged
[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
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Backport of #128217 to release/10.0
Customer Impact
  •  Customer reported
  •  Found internally
GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.
Regression
  •  Yes, introduced when GC regions were enabled
  •  No
Testing
CI tests, local testing using targeted repro app from the customer, GC tests
Risk
Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.
@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli
added this to the 10.0.x milestone
May 18, 2026
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from VSadovMay 18, 2026 22:25
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Pull request overview
This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.
Changes:
  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).
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cc: @BenV
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May 19, 2026•

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cc: @BenV
Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!
Edit: The stress tests passed full 4 hour runs on both of my test machines!
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Fixes #127687.
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@janvorli, once it is code reviewed, we can merge.
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LGTM. Thanks!
@JulieLeeMSFT
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merged commit da8ffe6
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dotnet:release/10.0May 20, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@BenV
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May 26, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@janvorli is there a good way to track which 10.0.x release this will land in?
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May 26, 2026

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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.
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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.
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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.
@snakefoot
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May 26, 2026•

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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?
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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
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} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

Skip to content
[release/10.0] Fix heap_segment_used watermark after compaction - #128342
Merged
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
May 20, 2026
Merged
[release/10.0] Fix heap_segment_used watermark after compaction#128342
JulieLeeMSFT merged 1 commit into
dotnet:release/10.0from 
janvorli:backport-10-gc-compact-phase-used-watermark
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@janvorli
@janvorlijanvorli
commented
May 18, 2026

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Backport of #128217 to release/10.0
Customer Impact
  •  Customer reported
  •  Found internally
GC with large pages enabled in regions mode can lead to intermittent crashes due to non-zeroed memory being returned for an allocation request that expects the memory to be zeroed.
Regression
  •  Yes, introduced when GC regions were enabled
  •  No
Testing
CI tests, local testing using targeted repro app from the customer, GC tests
Risk
Low. It adds maintaining heap_segment_used watermark after compaction so that it covers all the touched memory in a region. Before this change, it was stale (lower) for regions that receive relocated objects.
@janvorli
Fix heap_segment_used watermark after compaction
2554c10
After compact_phase, heap_segment_used can be stale — lower than
the actual end of live data - because `plan_phase` sets
`plan_allocated` beyond used for regions that receive relocated objects.
When `decommit_region` later clears memory only up to used instead of
committed (the large-pages / never_decommit_p path),
the gap between used and plan_allocated retains dirty data from
a previous region lifetime, causing heap corruption on the next GC cycle.
Fix:
At the end of `compact_phase`, bump heap_segment_used to
`max(used, plan_allocated)` for every non-read-only region in
the condemned generations and one generation above (the maximum
compaction target range).
The fix cost is zero when no compaction occurs. When compaction
does occur, it avoids unnecessary `memclr` in `decommit_region`
by keeping the used watermark accurate, so only truly unused memory
is cleared.
@janvorlijanvorli
added this to the 10.0.x milestone
May 18, 2026
@janvorli
janvorli
requested a review
from VSadovMay 18, 2026 22:25
@janvorlijanvorli
self-assigned this
May 18, 2026
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Pull request overview
This backport addresses a GC regions + large pages correctness issue where stale heap_segment_used values after compaction could cause decommit_region to clear an insufficient range, allowing dirty memory to be reused and leading to heap corruption/crashes.
Changes:
  • In compact_phase (regions mode), updates each affected region’s heap_segment_used to cover max(used, plan_allocated) for condemned generations and one generation above.
  • In decommit_heap_segment (regions mode), skips decommitting when large pages are enabled to avoid incorrect “logical decommit” behavior (large-page decommit is a no-op).
@janvorli
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May 18, 2026

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cc: @BenV
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May 19, 2026•

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cc: @BenV
Just wanted to confirm that this appears to resolve the issue on .NET 10 as expected. I'll keep running the tests for the full 4 hours just in case and report back. Thanks again for all your support @janvorli@cshung@mangod9, we really appreciate it!
Edit: The stress tests passed full 4 hour runs on both of my test machines!
@JulieLeeMSFT
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Fixes #127687.
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@janvorli, once it is code reviewed, we can merge.
VSadov
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Comment threadsrc/coreclr/gc/gc.cpp
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LGTM. Thanks!
@JulieLeeMSFT
JulieLeeMSFT
merged commit da8ffe6
into
dotnet:release/10.0May 20, 2026

126 of 129 checks passed

@dotnet-maestrodotnet-maestroBot
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May 20, 2026
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Guess it should be assigned the upcoming milestone 10.0.9 ?
@BenV
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May 26, 2026

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Guess it should be assigned the upcoming milestone 10.0.9 ?
@janvorli is there a good way to track which 10.0.x release this will land in?
@janvorli
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May 26, 2026

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@BenV it should be part of .NET 10.0.10, last 10.0.9 change was merged on May 13. I have originally thought it would make it to 10.0.9, but found it was not the case.
@snakefoot
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May 26, 2026

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@janvorli But #127573 has milestone 10.0.9 and was merged a week ago. Microsoft is running a very strange ship.
@janvorli
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May 26, 2026

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@snakefoot that change will also be in .NET 10.0.10. It was planned to get to 10.0.9, but we were waiting for customer verification of that change and missed the deadline for 10.0.9.
@snakefoot
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May 26, 2026•

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I understand that missing milestones is a common scenario. It is more the Microsoft way of updating the milestone for their pull-requests. So maybe better to give up and always rely on 10.x ?
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