From: Kairui Song kasong@tencent.com
The order check and fallback loop is updating the index value on every loop, this will cause the index to be wrongly 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@vger.kernel.org Link: https://lore.kernel.org/linux-mm/CAMgjq7DqgAmj25nDUwwu1U2cSGSn8n4-Hqpgottedy... [1] Fixes: e7a2ab7b3bb5d ("mm: shmem: add mTHP support for anonymous shmem") Signed-off-by: Kairui Song kasong@tencent.com
---
Changes from V2: - Introduce a temporary variable to improve code, no behavior change, generated code is identical. - Link to V2: https://lore.kernel.org/linux-mm/20251022105719.18321-1-ryncsn@gmail.com/
Changes from V1: - Remove unnecessary cleanup and simplify the commit message. - Link to V1: https://lore.kernel.org/linux-mm/20251021190436.81682-1-ryncsn@gmail.com/
--- mm/shmem.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/mm/shmem.c b/mm/shmem.c index b50ce7dbc84a..e1dc2d8e939c 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1882,6 +1882,7 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf, struct shmem_inode_info *info = SHMEM_I(inode); unsigned long suitable_orders = 0; struct folio *folio = NULL; + pgoff_t aligned_index; long pages; int error, order;
@@ -1895,10 +1896,12 @@ 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) + aligned_index = round_down(index, pages); + folio = shmem_alloc_folio(gfp, order, info, aligned_index); + if (folio) { + index = aligned_index; goto allocated; + }
if (pages == HPAGE_PMD_NR) count_vm_event(THP_FILE_FALLBACK);