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mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths.patch removed from -mm tree

merged mm-stable Andrew Morton 1 participant 1 message Oct 24 Stable branch

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The quilt patch titled
Subject: mm/memfd: fix hugetlb reservation accounting in error paths
has been removed from the -mm tree. Its filename was
mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Hongfu Li <[email protected]>
Subject: mm/memfd: fix hugetlb reservation accounting in error paths
Date: Thu, 3 Sep 2026 11:01:34 +0800

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.

Link https://lore.kernel.org/[email protected] open here
Link https://lore.kernel.org/[email protected] open here
Fixes 717cf9357325 ("mm/memfd: reserve hugetlb folios before allocation")
Signed-off-by Hongfu Li <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
Acked-by David Hildenbrand (Arm) <[email protected]>
Cc Baolin Wang <[email protected]>
Cc Hugh Dickins <[email protected]>
Cc Muchun Song <[email protected]>
Cc Oscar Salvador <[email protected]>
Cc Vivek Kasireddy <[email protected]>
Cc <[email protected]>

---

 mm/memfd.c |   31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)
mm/memfd.c renamed from mm/memfd.c~mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths +16 -15
@@ -82,22 +82,31 @@ struct folio *memfd_alloc_folio(struct f
 		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 f
 			 */
 			__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 f
 
 			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

_

Patches currently in -mm which might be from [email protected] are