Forwarded: [PATCH v4] hugetlbfs: check for shareable lock before calling huge_pmd_unshare()

syzbot
2025-10-07T06:01:55Z

Subject: [PATCH v4] hugetlbfs: check for shareable lock before calling huge_pmd_unshare()
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

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 skipping VMAs or adding new flags, check __vma_shareable_lock()
directly in __unmap_hugepage_range() right before calling huge_pmd_unshare().
This ensures PMD unsharing only happens when the VMA has a shareable lock
structure, while still allowing page unmapping and freeing to proceed for
all VMAs.

Reported-by: syzbot+f26d7c75c26ec19790e7@syzkaller.appspotmail.com
Reported-by: Mark Brown <broonie@kernel.org>
Fixes: dd83609b8898 ("hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list")
Suggested-by: Oscar Salvador <osalvador@suse.de>
Suggested-by: David Hildenbrand <david@redhat.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]
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
Changes in v4:
- Simplified approach per Oscar's suggestion: check __vma_shareable_lock()
directly in __unmap_hugepage_range() before calling huge_pmd_unshare()
- Removed ZAP_FLAG_NO_UNSHARE flag per David's feedback to avoid polluting
generic mm.h header
- Reverted hugetlb_vmdelete_list() to not skip VMAs

Changes in v3:
- Added ZAP_FLAG_NO_UNSHARE to skip only PMD unsharing, not entire VMA

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

Changes in v1:
- Initial fix attempt
---
fs/hugetlbfs/inode.c | 10 +---------
mm/hugetlb.c | 2 +-
2 files changed, 2 insertions(+), 10 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 9c94ed8c3ab0..1e040db18b20 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 +488,7 @@ 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 6cac826cb61f..9ed85ab8420e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5885,7 +5885,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
}

ptl = huge_pte_lock(h, mm, ptep);
- if (huge_pmd_unshare(mm, vma, address, ptep)) {
+ if (__vma_shareable_lock(vma) && huge_pmd_unshare(mm, vma, address, ptep)) {
spin_unlock(ptl);
tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE);
force_flush = true;
--
2.43.0