linux-kernel - [PATCH v2] mm/shmem: fix THP allocation and fallback loop

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Message-ID: <20251022105719.18321-1-ryncsn@gmail.com>
Date: Wed, 22 Oct 2025 18:57:19 +0800
From: Kairui Song <ryncsn@...il.com>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Hugh Dickins <hughd@...gle.com>,
	Dev Jain <dev.jain@....com>,
	David Hildenbrand <david@...hat.com>,
	Barry Song <baohua@...nel.org>,
	Liam Howlett <liam.howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Mariano Pache <npache@...hat.com>,
	Matthew Wilcox <willy@...radead.org>,
	Ryan Roberts <ryan.roberts@....com>,
	Zi Yan <ziy@...dia.com>,
	linux-kernel@...r.kernel.org,
	Kairui Song <kasong@...cent.com>,
	stable@...r.kernel.org
Subject: [PATCH v2] mm/shmem: fix THP allocation and fallback loop

From: Kairui Song <kasong@...cent.com>

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].

Cc: stable@...r.kernel.org
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@...cent.com>
---

Changes from V1:
- Link to V1: https://lore.kernel.org/linux-mm/20251021190436.81682-1-ryncsn@gmail.com/
- Remove unnecessary cleanup and simplify the commit message.

 mm/shmem.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index b50ce7dbc84a..7559773ebb30 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1895,10 +1895,11 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
 		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);
-- 
2.51.0