Skip decommit for large pages and add fake large pages test mode#127290
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janvorli merged 3 commits intodotnet:mainfrom Apr 28, 2026
Merged
Skip decommit for large pages and add fake large pages test mode#127290janvorli merged 3 commits intodotnet:mainfrom
janvorli merged 3 commits intodotnet:mainfrom
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mangod9
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The test fails on x86. Perhaps just make the test incompatible with 32bit? |
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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
Address review feedback from mangod9 and janvorli.
Rename large_pages_fake_mode_p to large_pages_emulation_mode_p and update comments to use emulation terminology throughout. Disable test on 32-bit: GCHeapHardLimit=0xC0000000 exceeds the virtual address space and GCLargePages is gated by HOST_64BIT.
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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