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Commit60fbb14396("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") return false unconditionally after split_huge_pmd_locked(). This may fail try_to_migrate() early when TTU_SPLIT_HUGE_PMD is specified. The reason is the above commit adjusted try_to_migrate_one() to, when a PMD-mapped THP entry is found, and TTU_SPLIT_HUGE_PMD is specified (for example, via unmap_folio()), return false unconditionally. This breaks the rmap walk and fail try_to_migrate() early, if this PMD-mapped THP is mapped in multiple processes. The user sensible impact of this bug could be: * On memory pressure, shrink_folio_list() may split partially mapped folio with split_folio_to_list(). Then free unmapped pages without IO. If failed, it may not be reclaimed. * On memory failure, memory_failure() would call try_to_split_thp_page() to split folio contains the bad page. If succeed, the PG_has_hwpoisoned bit is only set in the after-split folio contains @split_at. By doing so, we limit bad memory. If failed to split, the whole folios is not usable. One way to reproduce: Create an anonymous THP range and fork 512 children, so we have a THP shared mapped in 513 processes. Then trigger folio split with /sys/kernel/debug/split_huge_pages debugfs to split the THP folio to order 0. Without the above commit, we can successfully split to order 0. With the above commit, the folio is still a large folio. And currently there are two core users of TTU_SPLIT_HUGE_PMD: * try_to_unmap_one() * try_to_migrate_one() try_to_unmap_one() would restart the rmap walk, so only try_to_migrate_one() is affected. We can't simply revert commit60fbb14396("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()"), since it removed some duplicated check covered by page_vma_mapped_walk(). This patch fixes this by restart page_vma_mapped_walk() after split_huge_pmd_locked(). Since we cannot simply return "true" to fix the problem, as that would affect another case: When invoking folio_try_share_anon_rmap_pmd() from split_huge_pmd_locked(), the latter can fail and leave a large folio mapped through PTEs, in which case we ought to return true from try_to_migrate_one(). This might result in unnecessary walking of the rmap but is relatively harmless. Link: https://lkml.kernel.org/r/20260305015006.27343-1-richard.weiyang@gmail.com Fixes:60fbb14396("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") Signed-off-by: Wei Yang <richard.weiyang@gmail.com> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com> Reviewed-by: Zi Yan <ziy@nvidia.com> Tested-by: Lance Yang <lance.yang@linux.dev> Reviewed-by: Lance Yang <lance.yang@linux.dev> Reviewed-by: Gavin Guo <gavinguo@igalia.com> Acked-by: David Hildenbrand (arm) <david@kernel.org> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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