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

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

[Andrew Morton]

The patch titled
     Subject: mm/shmem: fix THP allocation and fallback loop
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-shmem-fix-thp-allocation-and-fallback-loop.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-and-fallback-loop.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 and fallback loop
Date: Wed, 22 Oct 2025 18:57:19 +0800

The order check and fallback loop is updating the index value on every
loop, this will cause the index to be aligned by a larger value while the
loop shrinks the order.

This may result in inserting and returning a folio of the wrong index and
cause data corruption with some userspace workloads [1].

Link: https://lkml.kernel.org/r/20251022105719.18321-1-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 |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/mm/shmem.c~mm-shmem-fix-thp-allocation-and-fallback-loop
+++ a/mm/shmem.c
@@ -1895,10 +1895,11 @@ static struct folio *shmem_alloc_and_add
 		order = highest_order(suitable_orders);
 		while (suitable_orders) {
 			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);
_

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

mm-shmem-fix-thp-allocation-and-fallback-loop.patch