fix(memory): use fallocate(PUNCH_HOLE) for discard_range on MAP_SHARED regions - #6168
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fix(memory): use fallocate(PUNCH_HOLE) for discard_range on MAP_SHARED regions#6168AdaAibaby wants to merge 1 commit into
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…D regions madvise(MADV_DONTNEED) has no effect on MAP_SHARED file-backed mappings (such as memfd regions used for huge-pages guest memory): the kernel ignores it for shared pages because other mappers may still be using them. As a result, balloon inflation never actually freed host physical frames when huge pages were enabled. Add a dedicated match arm in GuestRegionMmapExt::discard_range() for MAP_SHARED mappings that uses fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE) instead. This punches a hole in the backing file, causing the kernel to release the physical frames while keeping the virtual mapping intact; subsequent guest accesses fault in fresh zero pages, matching the expected balloon-inflation semantics. The existing MAP_PRIVATE (snapshot restore) and anonymous arms are unchanged. Add test_discard_range_on_memfd to verify the new path actually zeroes the discarded page. Fixes firecracker-microvm#6167
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Summary
GuestRegionMmapExt::discard_range()was callingmadvise(MADV_DONTNEED)for all non-private-file-backed mappings. For anonymous regions this works
correctly, but for
MAP_SHAREDmemfd-backed regions (used when huge pagesare enabled) the kernel ignores
MADV_DONTNEEDfor shared pages and returns0 — so balloon inflation never freed any host physical memory.
This PR adds a dedicated match arm for
MAP_SHAREDmappings that callsfallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE)instead, whichpunches a hole in the backing memfd file and releases the physical frames.
Subsequent guest accesses fault in fresh zero pages, matching the expected
balloon-inflation semantics.
Changes
src/vmm/src/vstate/memory.rs: addMAP_SHAREDarm toGuestRegionMmapExt::discard_range()usingfallocate(PUNCH_HOLE)test_discard_range_on_memfdthat writes data, callsdiscard_range,and asserts the discarded page reads back as zeroes
Testing
The new test
test_discard_range_on_memfdcovers the happy path (page iszeroed after discard), the second page is unaffected, out-of-range errors,
and the unaligned-offset error path.
Fixes #6167
Hi @Manciukic @JackThomson2 @JamesC1305 Would you mind taking a look when you have a chance? Thanks!