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fix for largepages with agressive decommit logic #126929

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Merged
mangod9 merged 3 commits into from
Apr 16, 2026
Merged

fix for largepages with agressive decommit logic #126929

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adc1262
fix for largepages with agressive decommit
mangod9 Apr 15, 2026
3a643c5
CR Feedback
mangod9 Apr 15, 2026
009ad4d
adding null check for clarity
mangod9 Apr 15, 2026
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2 changes: 1 addition & 1 deletion src/coreclr/gc/gcpriv.h
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Original file line number Diff line number Diff line change
Expand Up @@ -2485,7 +2485,7 @@ class gc_heap
PER_HEAP_METHOD void decommit_heap_segment (heap_segment* seg);
PER_HEAP_ISOLATED_METHOD bool virtual_alloc_commit_for_heap (void* addr, size_t size, int h_number);
PER_HEAP_ISOLATED_METHOD bool virtual_commit (void* address, size_t size, int bucket, int h_number=-1, bool* hard_limit_exceeded_p=NULL);
PER_HEAP_ISOLATED_METHOD bool virtual_decommit (void* address, size_t size, int bucket, int h_number=-1);
PER_HEAP_ISOLATED_METHOD bool virtual_decommit (void* address, size_t size, int bucket, int h_number=-1, void* end_of_data=nullptr);
PER_HEAP_ISOLATED_METHOD void reduce_committed_bytes (void* address, size_t size, int bucket, int h_number, bool decommit_succeeded_p);
friend void destroy_card_table (uint32_t*);
PER_HEAP_ISOLATED_METHOD void destroy_card_table_helper (uint32_t* c_table);
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10 changes: 9 additions & 1 deletion src/coreclr/gc/memory.cpp
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Expand Up @@ -161,7 +161,7 @@ void gc_heap::reduce_committed_bytes (void* address, size_t size, int bucket, in
}
}

bool gc_heap::virtual_decommit (void* address, size_t size, int bucket, int h_number)
bool gc_heap::virtual_decommit (void* address, size_t size, int bucket, int h_number, void* end_of_data)
{
/**
* Here are all possible cases for the decommits:
Expand All @@ -173,6 +173,14 @@ bool gc_heap::virtual_decommit (void* address, size_t size, int bucket, int h_nu

bool decommit_succeeded_p = ((bucket != recorded_committed_bookkeeping_bucket) && use_large_pages_p) ? true : GCToOSInterface::VirtualDecommit (address, size);

// Large pages: the decommit above is a no-op so memory retains stale data.
// Clear up to end_of_data if the caller provided it so that the heap never
// observes leftover object references after the region is reused.
if (use_large_pages_p && (end_of_data != nullptr) && (end_of_data > address))
{
memclr ((uint8_t*)address, (uint8_t*)end_of_data - (uint8_t*)address);
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@jkotas jkotas Apr 16, 2026

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In other paths, the GC just takes keeps track of the fact that memory is dirty and clears it right before it is used for allocations again in gc_heap::adjust_limit_clr. Would it be a better option here?

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@mangod9 mangod9 Apr 16, 2026

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the fix was following the same pattern like this in decommit_region:

runtime/src/coreclr/gc/memory.cpp

Lines 358 to 373 in 75d3e60

if (require_clearing_memory_p)
{
uint8_t* clear_end = use_large_pages_p ? heap_segment_used (region) : heap_segment_committed (region);
size_t clear_size = clear_end - page_start;
memclr (page_start, clear_size);
heap_segment_used (region) = heap_segment_mem (region);
dprintf(REGIONS_LOG, ("cleared region %p(%p-%p) (%zu bytes)",
region,
page_start,
clear_end,
clear_size));
}
else
{
heap_segment_committed (region) = heap_segment_mem (region);
}

where memclr clears the full region for large_pages. Similar cleanup was missing during aggressive decommitting of tail regions.

}

reduce_committed_bytes (address, size, bucket, h_number, decommit_succeeded_p);

return decommit_succeeded_p;
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2 changes: 1 addition & 1 deletion src/coreclr/gc/regions_segments.cpp
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Expand Up @@ -1834,7 +1834,7 @@ void gc_heap::distribute_free_regions()
size_t end_space = heap_segment_committed (region) - aligned_allocated;
if (end_space > 0)
{
virtual_decommit (aligned_allocated, end_space, gen_to_oh (i), hn);
virtual_decommit (aligned_allocated, end_space, gen_to_oh (i), hn, heap_segment_used (region));
heap_segment_committed (region) = aligned_allocated;
heap_segment_used (region) = min (heap_segment_used (region), heap_segment_committed (region));
assert (heap_segment_committed (region) > heap_segment_mem (region));
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