[PATCH v8] hugetlbfs: move lock assertions after early returns in huge_pmd_unshare()

[Deepanshu Kartikey <kartikey406@gmail.com>]

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 approach reverts the VMA skipping logic from commit dd83609b8898
("hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list")
while moving the assertions to avoid the assertion failure, keeping all the
logic within huge_pmd_unshare() itself and allowing page unmapping and
freeing to proceed for all VMAs.

Reported-by: syzbot+f26d7c75c26ec19790e7@syzkaller.appspotmail.com
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://syzkaller.appspot.com/bug?extid=f26d7c75c26ec19790e7
Fixes: dd83609b8898 ("hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list")
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Oscar Salvador <osalvador@suse.de>
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]
Link: https://lore.kernel.org/linux-mm/CADhLXY72yEVDjXWfxBUXfXhNfb8MWqwJmcb1daEHmDeFW+DRGw@mail.gmail.com/ [v5]
Link: https://lore.kernel.org/linux-mm/e6bb05f7-8f05-409f-9d87-2d25f66942a9@redhat.com/ [v6]
Link: https://lore.kernel.org/linux-mm/CADhLXY4WPxzvzuiZPJmhS-9xMqRZ_qf7ZcFf5MXPgXbgB3_Xzg@mail.gmail.com/ [v7]
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
Changes in v8:
- Add missing revert of skip code in fs/hugetlbfs/inode.c that was
  introduced in commit dd83609b8898
- Move lock assertions after early returns in huge_pmd_unshare()
- Based on mm/mm-stable

Changes in v7:
- Rebased on mm/mm-stable per David's request
- Forgot to revert skip code from fs/hugetlbfs/inode.c

Changes in v6:
- Remove __vma_shareable_lock() check from __unmap_hugepage_range()
- Move lock assertions after early returns in huge_pmd_unshare()

Changes in v5:
- Incomplete: only moved assertions, forgot to remove v4 check

Changes in v4:
- Check __vma_shareable_lock() in __unmap_hugepage_range() before calling
  huge_pmd_unshare() per Oscar's suggestion

Changes in v3:
- Add ZAP_FLAG_NO_UNSHARE to skip only PMD unsharing

Changes in v2:
- Skip entire VMAs without shareable locks (caused PUNCH_HOLE regression)

Changes in v1:
- Initial fix attempt
---
 fs/hugetlbfs/inode.c | 9 ---------
 mm/hugetlb.c         | 5 ++---
 2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 9c94ed8c3ab0..f42548ee9083 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -478,14 +478,6 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
 		if (!hugetlb_vma_trylock_write(vma))
 			continue;
 
-		/*
-		 * Skip VMAs without shareable locks. Per the design in commit
-		 * 40549ba8f8e0, these will be handled by remove_inode_hugepages()
-		 * called after this function with proper locking.
-		 */
-		if (!__vma_shareable_lock(vma))
-			goto skip;
-
 		v_start = vma_offset_start(vma, start);
 		v_end = vma_offset_end(vma, end);
 
@@ -496,7 +488,6 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
 		 * vmas.  Therefore, lock is not held when calling
 		 * unmap_hugepage_range for private vmas.
 		 */
-skip:
 		hugetlb_vma_unlock_write(vma);
 	}
 }
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