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

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Message-ID: <88d5ad81-04b0-4f2e-8099-807f1d32e54e@lucifer.local>
Date: Fri, 24 Oct 2025 15:02:36 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, 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>,
        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: Re: [PATCH v3] mm/shmem: fix THP allocation and fallback loop

On Thu, Oct 23, 2025 at 02:59:13PM +0800, Kairui Song wrote:
> 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 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@...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>

Yikes... LGTM so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

See below for a small nit.

>
> ---
>
> 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;

Nit, but can't we just declare this in the loop? That makes it even clearer
that we don't reuse the value.

>  	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);
> --
> 2.51.0
>