Re: [PATCH v4] mm/userfaultfd: fix hugetlb fault mutex hash calculation

[Andrew Morton]

On Wed, 11 Mar 2026 18:54:26 +0800 Jianhui Zhou <jianhuizzzzz@xxxxxxxxx> wrote:

> On Tue, Mar 10, 2026 at 12:47:07PM -0700, jane.chu@xxxxxxxxxx wrote:
> > Just wondering whether making the shift explicit here instead of
> > introducing another hugetlb helper might be sufficient?
> >
> >      idx >>= huge_page_order(hstate_vma(vma));
> 
> That would work for hugetlb VMAs since both (address - vm_start) and
> vm_pgoff are guaranteed to be huge page aligned. However, David
> suggested introducing hugetlb_linear_page_index() to provide a cleaner
> API that mirrors linear_page_index(), so I kept this approach.
> 

Thanks.

Would anyone like to review this cc:stable patch for us?


From: Jianhui Zhou <jianhuizzzzz@xxxxxxxxx>
Subject: mm/userfaultfd: fix hugetlb fault mutex hash calculation
Date: Tue, 10 Mar 2026 19:05:26 +0800

In mfill_atomic_hugetlb(), linear_page_index() is used to calculate the
page index for hugetlb_fault_mutex_hash().  However, linear_page_index()
returns the index in PAGE_SIZE units, while hugetlb_fault_mutex_hash()
expects the index in huge page units.  This mismatch means that different
addresses within the same huge page can produce different hash values,
leading to the use of different mutexes for the same huge page.  This can
cause races between faulting threads, which can corrupt the reservation
map and trigger the BUG_ON in resv_map_release().

Fix this by introducing hugetlb_linear_page_index(), which returns the
page index in huge page granularity, and using it in place of
linear_page_index().

Link: https://lkml.kernel.org/r/20260310110526.335749-1-jianhuizzzzz@xxxxxxxxx
Fixes: a08c7193e4f1 ("mm/filemap: remove hugetlb special casing in filemap.c")
Signed-off-by: Jianhui Zhou <jianhuizzzzz@xxxxxxxxx>
Reported-by: syzbot+f525fd79634858f478e7@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=f525fd79634858f478e7
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: JonasZhou <JonasZhou@xxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: SeongJae Park <sj@xxxxxxxxxx>
Cc: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 include/linux/hugetlb.h |   17 +++++++++++++++++
 mm/userfaultfd.c        |    2 +-
 2 files changed, 18 insertions(+), 1 deletion(-)

--- a/include/linux/hugetlb.h~mm-userfaultfd-fix-hugetlb-fault-mutex-hash-calculation
+++ a/include/linux/hugetlb.h
@@ -796,6 +796,23 @@ static inline unsigned huge_page_shift(s
 	return h->order + PAGE_SHIFT;
 }
 
+/**
+ * hugetlb_linear_page_index() - linear_page_index() but in hugetlb
+ *				 page size granularity.
+ * @vma: the hugetlb VMA
+ * @address: the virtual address within the VMA
+ *
+ * Return: the page offset within the mapping in huge page units.
+ */
+static inline pgoff_t hugetlb_linear_page_index(struct vm_area_struct *vma,
+		unsigned long address)
+{
+	struct hstate *h = hstate_vma(vma);
+
+	return ((address - vma->vm_start) >> huge_page_shift(h)) +
+		(vma->vm_pgoff >> huge_page_order(h));
+}
+
 static inline bool order_is_gigantic(unsigned int order)
 {
 	return order > MAX_PAGE_ORDER;
--- a/mm/userfaultfd.c~mm-userfaultfd-fix-hugetlb-fault-mutex-hash-calculation
+++ a/mm/userfaultfd.c
@@ -573,7 +573,7 @@ retry:
 		 * in the case of shared pmds.  fault mutex prevents
 		 * races with other faulting threads.
 		 */
-		idx = linear_page_index(dst_vma, dst_addr);
+		idx = hugetlb_linear_page_index(dst_vma, dst_addr);
 		mapping = dst_vma->vm_file->f_mapping;
 		hash = hugetlb_fault_mutex_hash(mapping, idx);
 		mutex_lock(&hugetlb_fault_mutex_table[hash]);
_