+ mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths.patch added to mm-unstable branch

+ mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/memfd: fix hugetlb reservation accounting in error paths
has been added to the -mm mm-unstable branch.  Its filename is
     mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Hongfu Li <lihongfu@kylinos.cn>
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/20260927094757.31665-1-hongfu.li@linux.dev
Link: https://lore.kernel.org/20260903030134.7407-1-hongfu.li@linux.dev
Fixes: 717cf9357325 ("mm/memfd: reserve hugetlb folios before allocation")
Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Cc: <stable@vger.kernel.org>
---

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

--- a/mm/memfd.c~mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths
+++ a/mm/memfd.c
@@ -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 lihongfu@kylinos.cn are

mm-use-a-folio-in-the-softleaf_is_device_private-path.patch
mm-hugetlb-fix-resv_huge_pages-double-decrement-in-memfd-error-path.patch
mm-memcontrol-remove-unused-memcg-parameter-in-calculate_high_delay.patch
selftests-mm-khugepaged-consolidate-error-exits-via-kselftest-helpers.patch
mm-page_owner-preserve-original-free_pid-free_tgid-during-folio-migration.patch
mm-memfd-fix-hugetlb-reservation-accounting-in-error-paths.patch
selftests-mm-fix-ptrace-peekdata-check-in-memfd_secret-test.patch
mm-hugetlb_cgroup-move-per-node-usage-on-cross-node-migration.patch
mm-hugetlb_cgroup-move-per-node-usage-on-cgroup-reparenting.patch
mm-hugetlb-account-migration-target-folio-in-per-node-nr_hugetlb-vmstat.patch
mm-memcg-migrate-per-node-hugetlb-lruvec-stat-together-with-hugetlb-folio.patch