Skip decommit for large pages and add fake large pages test mode#127290
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cshung wants to merge 1 commit intodotnet:mainfrom
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Skip decommit for large pages and add fake large pages test mode#127290cshung wants to merge 1 commit intodotnet:mainfrom
cshung wants to merge 1 commit intodotnet:mainfrom
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With large pages, VirtualDecommit is a no-op since large pages cannot be partially decommitted. PR dotnet#126929 fixed the resulting stale data corruption by adding memclr in virtual_decommit, but this approach has downsides: the memory is never returned to the OS, yet we pay for the clearing and produce misleading committed/used bookkeeping. Instead, skip the decommit entirely for large pages: 1. distribute_free_regions: skip the aggressive tail-region decommit (the committed-but-unallocated tail of in-use regions). This was the path that caused the heap corruption in dotnet#126903. 2. decommit_heap_segment: skip the whole-segment decommit used for segment hoarding and BGC segment deletion. Same class of issue: committed/used are lowered but physical memory retains stale data. 3. decommit_region: bypass virtual_decommit and call reduce_committed_bytes directly, since decommit_region already handles large pages correctly by clearing memory itself. 4. virtual_decommit: add an assert that it is never called for heap memory when large pages are on. This catches any future caller that forgets to handle the large pages case. The end_of_data parameter and no-op ternary added by dotnet#126929 are removed. Add GCLargePages=2 mode that simulates large pages using small pages: sets use_large_pages_p=true but reserves with normal pages and commits everything upfront. This exercises all large page GC code paths without requiring OS large page setup or privileges, enabling CI testing. Fix dotnet#126903
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Tagging subscribers to this area: @JulieLeeMSFT, @dotnet/gc |
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mangod9
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Apr 22, 2026
| heap_segment* gc_heap::segment_standby_list; | ||
| #endif //USE_REGIONS | ||
| bool gc_heap::use_large_pages_p = 0; | ||
| bool gc_heap::large_pages_fake_mode_p = 0; |
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probably better to name this large_pages_force_mode or something.
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I'd prefer large_pages_simulation_mode_p or large_pages_emulation_mode_p
mangod9
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Apr 22, 2026
| { | ||
| memclr ((uint8_t*)address, (uint8_t*)end_of_data - (uint8_t*)address); | ||
| } | ||
| bool decommit_succeeded_p = GCToOSInterface::VirtualDecommit (address, size); |
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think you need a similar fix in gc_heap::decommit_region ?
VSadov
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Apr 22, 2026
| // VirtualDecommit is a no-op for large pages so skip it and update | ||
| // committed bookkeeping directly. Memory clearing is handled below. | ||
| decommit_succeeded_p = true; | ||
| reduce_committed_bytes (page_start, decommit_size, bucket, h_number, true); |
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If decommit is a noop, why are we reducing comitted bytes?
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The test fails on x86. Perhaps just make the test incompatible with 32bit? |
This was referenced Apr 22, 2026
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With large pages, VirtualDecommit is a no-op since large pages cannot be partially decommitted. PR #126929 fixed the resulting stale data corruption by adding memclr in virtual_decommit, but this approach has downsides: the memory is never returned to the OS, yet we pay for the clearing and produce misleading committed/used bookkeeping.
Instead, skip the decommit entirely for large pages:
distribute_free_regions: skip the aggressive tail-region decommit (the committed-but-unallocated tail of in-use regions). This was the path that caused the heap corruption in GC heap corruption with GCLargePages #126903.
decommit_heap_segment: skip the whole-segment decommit used for segment hoarding and BGC segment deletion. Same class of issue: committed/used are lowered but physical memory retains stale data.
decommit_region: bypass virtual_decommit and call reduce_committed_bytes directly, since decommit_region already handles large pages correctly by clearing memory itself.
virtual_decommit: add an assert that it is never called for heap memory when large pages are on. This catches any future caller that forgets to handle the large pages case. The end_of_data parameter and no-op ternary added by fix for largepages with agressive decommit logic #126929 are removed.
Add GCLargePages=2 mode that simulates large pages using small pages: sets use_large_pages_p=true but reserves with normal pages and commits everything upfront. This exercises all large page GC code paths without requiring OS large page setup or privileges, enabling CI testing.
Fix #126903