+ mm-shmem-fix-thp-allocation-size-check-and-fallback.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/shmem: fix THP allocation size check and fallback
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-shmem-fix-thp-allocation-size-check-and-fallback.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-shmem-fix-thp-allocation-size-check-and-fallback.patch

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

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------------------------------------------------------
From: Kairui Song <kasong@tencent.com>
Subject: mm/shmem: fix THP allocation size check and fallback
Date: Wed, 22 Oct 2025 03:04:36 +0800

There are some problems with the code implementations of THP fallback. 
suitable_orders could be zero, and calling highest_order on a zero value
returns an overflowed size.  And the order check loop is updating the
index value on every loop which may cause the index to be aligned by a
larger value while the loop shrinks the order.  And it forgot to try order
0 after the final loop.

This is usually fine because shmem_add_to_page_cache ensures the shmem
mapping is still sane, but it might cause many potential issues like
allocating random folios into the random position in the map or return
-ENOMEM by accident.  This triggered some strange userspace errors [1],
and shouldn't have happened in the first place.

Link: https://lkml.kernel.org/r/20251021190436.81682-1-ryncsn@gmail.com
Link: https://lore.kernel.org/linux-mm/CAMgjq7DqgAmj25nDUwwu1U2cSGSn8n4-Hqpgottedy0S6YYeUw@mail.gmail.com/ [1]
Fixes: e7a2ab7b3bb5d ("mm: shmem: add mTHP support for anonymous shmem")
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/shmem.c |   26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

--- a/mm/shmem.c~mm-shmem-fix-thp-allocation-size-check-and-fallback
+++ a/mm/shmem.c
@@ -1824,6 +1824,9 @@ static unsigned long shmem_suitable_orde
 	unsigned long pages;
 	int order;
 
+	if (!orders)
+		return 0;
+
 	if (vma) {
 		orders = thp_vma_suitable_orders(vma, vmf->address, orders);
 		if (!orders)
@@ -1888,27 +1891,28 @@ static struct folio *shmem_alloc_and_add
 	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
 		orders = 0;
 
-	if (orders > 0) {
-		suitable_orders = shmem_suitable_orders(inode, vmf,
-							mapping, index, orders);
+	suitable_orders = shmem_suitable_orders(inode, vmf,
+						mapping, index, orders);
 
+	if (suitable_orders) {
 		order = highest_order(suitable_orders);
-		while (suitable_orders) {
+		do {
 			pages = 1UL << order;
-			index = round_down(index, pages);
-			folio = shmem_alloc_folio(gfp, order, info, index);
-			if (folio)
+			folio = shmem_alloc_folio(gfp, order, info, round_down(index, pages));
+			if (folio) {
+				index = round_down(index, pages);
 				goto allocated;
+			}
 
 			if (pages == HPAGE_PMD_NR)
 				count_vm_event(THP_FILE_FALLBACK);
 			count_mthp_stat(order, MTHP_STAT_SHMEM_FALLBACK);
 			order = next_order(&suitable_orders, order);
-		}
-	} else {
-		pages = 1;
-		folio = shmem_alloc_folio(gfp, 0, info, index);
+		} while (suitable_orders);
 	}
+
+	pages = 1;
+	folio = shmem_alloc_folio(gfp, 0, info, index);
 	if (!folio)
 		return ERR_PTR(-ENOMEM);
 
_

Patches currently in -mm which might be from kasong@tencent.com are

mm-shmem-fix-thp-allocation-size-check-and-fallback.patch