Re: [PATCH v3] mm/shmem: fix THP allocation and fallback loop

[David Hildenbrand <david@redhat.com>]

On 23.10.25 08:59, Kairui Song wrote:
> 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-Hqpgottedy0S6YYeUw@mail.gmail.com/ [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;
> +			}

Was the found by code inspection or was there a report about this?

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb