mm/memfd: fix hugetlb reservation accounting in error paths

From: Hongfu Li <[email protected]>

If hugetlb_add_to_page_cache() in memfd_alloc_folio() fails with -EEXIST, a concurrent fault has already instantiated the folio in the page cache, and the reservation now belongs to that folio. Calling hugetlb_unreserve_pages() in that case incorrectly removes the region backing the cached folio and releases one reservation more than it should, leaving that folio in the page cache with no region recording it.

That leaves resv_huge_pages one page short until that folio is removed from the page cache, so available_huge_pages() reports a page that is not actually free and the pool can grant one reservation more than it can back. Applications using hugetlb memfds can then fail to allocate or fault in a page they already reserved. They see -ENOMEM or -ENOSPC from the allocation or fault path even though the reservation and HugePages_Free still look healthy.

So hold the hugetlb fault mutex from hugetlb_reserve_pages() until the error-path unreserve completes to make the reserve, allocate and instantiate steps atomic against concurrent faults. With the mutex held from the start, a concurrent fault can no longer consume the reservation between reserve and allocate/instantiate. If a fault completed before the mutex was taken, it has already added the region for that index, so hugetlb_reserve_pages() returns 0 and the error path leaves the region in place.

Fixes 717cf9357325 ("mm/memfd: reserve hugetlb folios before allocation")

Cc [email protected]

Signed-off-by Hongfu Li <[email protected]>

v3:
- Update commit message to describe the accounting damage and its user-visible effect; no code changes.
- Add Cc: [email protected].
v2:
- Take the hugetlb fault mutex before hugetlb_reserve_pages() and hold it until the error-path unreserve completes.
- Update commit message
- Link to v1: https://lore.kernel.org/all/[email protected]/

---
 mm/memfd.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

mm/memfd.c +16 -15

index c708d92533f4..0f6fff004f5e 100644
@@ -82,22 +82,31 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 		struct hstate *h = hstate_file(memfd);
 		int err = -ENOMEM;
 		long nr_resv;
+		u32 hash;
 
 		gfp_mask = htlb_alloc_mask(h);
 		gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE);
 		idx >>= huge_page_order(h);
 
+		/*
+		 * Serialize hugepage allocation and instantiation to prevent
+		 * races with concurrent allocations, as required by all other
+		 * callers of hugetlb_add_to_page_cache().
+		 */
+		hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx);
+		mutex_lock(&hugetlb_fault_mutex_table[hash]);
+
 		nr_resv = hugetlb_reserve_pages(inode, idx, idx + 1, NULL, EMPTY_VMA_FLAGS);
-		if (nr_resv < 0)
-			return ERR_PTR(nr_resv);
+		if (nr_resv < 0) {
+			err = nr_resv;
+			goto out_unlock;
+		}
 
 		folio = alloc_hugetlb_folio_reserve(h,
 						    numa_node_id(),
 						    NULL,
 						    gfp_mask);
 		if (folio) {
-			u32 hash;
-
 			/*
 			 * Zero the folio to prevent information leaks to userspace.
 			 * Use folio_zero_user() which is optimized for huge/gigantic

@@ -112,20 +121,9 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 			 */
 			__folio_mark_uptodate(folio);
 
-			/*
-			 * Serialize hugepage allocation and instantiation to prevent
-			 * races with concurrent allocations, as required by all other
-			 * callers of hugetlb_add_to_page_cache().
-			 */
-			hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx);
-			mutex_lock(&hugetlb_fault_mutex_table[hash]);
-
 			err = hugetlb_add_to_page_cache(folio,
 							memfd->f_mapping,
 							idx);
-
-			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
-
 			if (err) {
 				folio_put(folio);
 				goto err_unresv;

@@ -133,11 +131,14 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 
 			hugetlb_set_folio_subpool(folio, subpool_inode(inode));
 			folio_unlock(folio);
+			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 			return folio;
 		}
 err_unresv:
 		if (nr_resv > 0)
 			hugetlb_unreserve_pages(inode, idx, idx + 1, 0);
+out_unlock:
+		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 		return ERR_PTR(err);
 	}
 #endif

Signature · 1 line

2.54.0

Quoted 26 lines · On Sun, 27 Sep 2026 17:47:57 +0800 Hongfu Li <[email protected]> wrote:

From: Hongfu Li <[email protected]>

If hugetlb_add_to_page_cache() in memfd_alloc_folio() fails with -EEXIST, a concurrent fault has already instantiated the folio in the page cache, and the reservation now belongs to that folio. Calling hugetlb_unreserve_pages() in that case incorrectly removes the region backing the cached folio and releases one reservation more than it should, leaving that folio in the page cache with no region recording it.

That leaves resv_huge_pages one page short until that folio is removed from the page cache, so available_huge_pages() reports a page that is not actually free and the pool can grant one reservation more than it can back. Applications using hugetlb memfds can then fail to allocate or fault in a page they already reserved. They see -ENOMEM or -ENOSPC from the allocation or fault path even though the reservation and HugePages_Free still look healthy.

So hold the hugetlb fault mutex from hugetlb_reserve_pages() until the error-path unreserve completes to make the reserve, allocate and instantiate steps atomic against concurrent faults. With the mutex held from the start, a concurrent fault can no longer consume the reservation between reserve and allocate/instantiate. If a fault completed before the mutex was taken, it has already added the region for that index, so hugetlb_reserve_pages() returns 0 and the error path leaves the region in place.

Quoted 3 lines

Fixes 717cf9357325 ("mm/memfd: reserve hugetlb folios before allocation")

Cc [email protected]

Signed-off-by Hongfu Li <[email protected]>

Can we please have review of this cc:stable regression fix?

Quoted 28 lines

v3:
- Update commit message to describe the accounting damage and its user-visible effect; no code changes.
- Add Cc: [email protected].
v2:
- Take the hugetlb fault mutex before hugetlb_reserve_pages() and hold it until the error-path unreserve completes.
- Update commit message
- Link to v1: https://lore.kernel.org/all/[email protected]/

---
 mm/memfd.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

mm/memfd.c +1 -0

index c708d92533f4..0f6fff004f5e 100644
@@ -82,22 +82,31 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 		struct hstate *h = hstate_file(memfd);
 		int err = -ENOMEM;
 		long nr_resv;
+		u32 hash;
 
 		gfp_mask = htlb_alloc_mask(h);
 		gfp_mask &= ~(__GFP_HIGHMEM | __GFP_MOVABLE);
 		idx >>= huge_page_order(h);
 

mm/memfd.c struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx) · line 96

Quoted 6 lines

mm/memfd.c +6 -0

+		/*
+		 * Serialize hugepage allocation and instantiation to prevent
+		 * races with concurrent allocations, as required by all other
+		 * callers of hugetlb_add_to_page_cache().
+		 */
+		hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx);

Quoted 43 lines

mm/memfd.c +6 -15

+		mutex_lock(&hugetlb_fault_mutex_table[hash]);
+
 		nr_resv = hugetlb_reserve_pages(inode, idx, idx + 1, NULL, EMPTY_VMA_FLAGS);
-		if (nr_resv < 0)
-			return ERR_PTR(nr_resv);
+		if (nr_resv < 0) {
+			err = nr_resv;
+			goto out_unlock;
+		}
 
 		folio = alloc_hugetlb_folio_reserve(h,
 						    numa_node_id(),
 						    NULL,
 						    gfp_mask);
 		if (folio) {
-			u32 hash;
-
 			/*
 			 * Zero the folio to prevent information leaks to userspace.
 			 * Use folio_zero_user() which is optimized for huge/gigantic

@@ -112,20 +121,9 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 			 */
 			__folio_mark_uptodate(folio);
 
-			/*
-			 * Serialize hugepage allocation and instantiation to prevent
-			 * races with concurrent allocations, as required by all other
-			 * callers of hugetlb_add_to_page_cache().
-			 */
-			hash = hugetlb_fault_mutex_hash(memfd->f_mapping, idx);
-			mutex_lock(&hugetlb_fault_mutex_table[hash]);
-
 			err = hugetlb_add_to_page_cache(folio,
 							memfd->f_mapping,
 							idx);
-
-			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
-
 			if (err) {
 				folio_put(folio);
 				goto err_unresv;

@@ -133,11 +131,14 @@ struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx)
 

mm/memfd.c struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx) · line 144

Quoted 13 lines

mm/memfd.c +3 -0

 			hugetlb_set_folio_subpool(folio, subpool_inode(inode));
 			folio_unlock(folio);
+			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 			return folio;
 		}
 err_unresv:
 		if (nr_resv > 0)
 			hugetlb_unreserve_pages(inode, idx, idx + 1, 0);
+out_unlock:
+		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 		return ERR_PTR(err);
 	}
 #endif