When hugetlb_vmdelete_list() processes VMAs during truncate operations, it may encounter VMAs where huge_pmd_unshare() is called without the required shareable lock. This triggers an assertion failure in hugetlb_vma_assert_locked(). The previous fix in commit dd83609b8898 ("hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list") skipped entire VMAs without shareable locks to avoid the assertion. However, this prevented pages from being unmapped and freed, causing a regression in fallocate(PUNCH_HOLE) operations where pages were not freed immediately, as reported by Mark Brown. Instead of checking locks in the caller or skipping VMAs, move the lock assertions in huge_pmd_unshare() to after the early return checks. The assertions are only needed when actual PMD unsharing work will be performed. If the function returns early because sz != PMD_SIZE or the PMD is not shared, no locks are required and assertions should not fire. This is cleaner than previous approaches because it keeps all the logic within huge_pmd_unshare() itself, while still allowing page unmapping and freeing to proceed for all VMAs. Reported-by: syzbot+f26d7c75c26ec19790e7@syzkaller.appspotmail.com Reported-by: Mark Brown Closes: https://syzkaller.appspot.com/bug?extid=f26d7c75c26ec19790e7 Fixes: dd83609b8898 ("hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list") Suggested-by: David Hildenbrand Suggested-by: Oscar Salvador Tested-by: syzbot+f26d7c75c26ec19790e7@syzkaller.appspotmail.com Link: https://lore.kernel.org/mm-commits/20250925203504.7BE02C4CEF7@smtp.kernel.org/ [v1] Link: https://lore.kernel.org/mm-commits/20250928185232.BEDB6C4CEF0@smtp.kernel.org/ [v2] Link: https://lore.kernel.org/linux-mm/20251003174553.3078839-1-kartikey406@gmail.com/ [v3] Link: https://lore.kernel.org/linux-mm/20251008052759.469714-1-kartikey406@gmail.com/ [v4] Signed-off-by: Deepanshu Kartikey --- Changes in v5: - Move lock assertions after early return checks in huge_pmd_unshare() per David's suggestion - cleaner approach that keeps logic within the function itself - Revert all previous approaches (VMA skipping, flag additions, caller checks) Changes in v4: - Check __vma_shareable_lock() in __unmap_hugepage_range() before calling huge_pmd_unshare() per Oscar's suggestion - Remove ZAP_FLAG_NO_UNSHARE flag per David's feedback Changes in v3: - Add ZAP_FLAG_NO_UNSHARE to skip only PMD unsharing, not entire VMA Changes in v2: - Skip entire VMAs without shareable locks (caused PUNCH_HOLE regression) Changes in v1: - Initial fix attempt --- mm/hugetlb.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 795ee393eac0..0455119716ec 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -7614,13 +7614,12 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma, p4d_t *p4d = p4d_offset(pgd, addr); pud_t *pud = pud_offset(p4d, addr); - i_mmap_assert_write_locked(vma->vm_file->f_mapping); - hugetlb_vma_assert_locked(vma); if (sz != PMD_SIZE) return 0; if (!ptdesc_pmd_is_shared(virt_to_ptdesc(ptep))) return 0; - + i_mmap_assert_write_locked(vma->vm_file->f_mapping); + hugetlb_vma_assert_locked(vma); pud_clear(pud); /* * Once our caller drops the rmap lock, some other process might be -- 2.34.1