linux-kernel - Re: [PATCH v3 05/15] mm, swap: always lock and check the swap cache folio before use

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Message-ID: <5617a501-3f46-423e-b3da-98efcd63d7c7@redhat.com>
Date: Fri, 12 Sep 2025 18:04:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Kairui Song <ryncsn@...il.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 Chris Li <chrisl@...nel.org>, Barry Song <baohua@...nel.org>,
 Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>,
 Kemeng Shi <shikemeng@...weicloud.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 linux-kernel@...r.kernel.org, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v3 05/15] mm, swap: always lock and check the swap cache
 folio before use

On 10.09.25 18:08, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
> 
> Swap cache lookup only increases the reference count of the returned
> folio. That's not enough to ensure a folio is stable in the swap
> cache, so the folio could be removed from the swap cache at any
> time. The caller should always lock and check the folio before using it.
> 
> We have just documented this in kerneldoc, now introduce a helper for swap
> cache folio verification with proper sanity checks.
> 
> Also, sanitize a few current users to use this convention and the new
> helper for easier debugging. They were not having observable problems
> yet, only trivial issues like wasted CPU cycles on swapoff or
> reclaiming. They would fail in some other way, but it is still better to
> always follow this convention to make things robust and make later
> commits easier to do.
> 
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---

[...]

> index 4baebd8b48f4..f1a4d381d719 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -240,14 +240,12 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
>   	 * Offset could point to the middle of a large folio, or folio
>   	 * may no longer point to the expected offset before it's locked.
>   	 */
> -	if (offset < swp_offset(folio->swap) ||
> -	    offset >= swp_offset(folio->swap) + nr_pages) {
> +	if (!folio_matches_swap_entry(folio, entry)) {
>   		folio_unlock(folio);
>   		folio_put(folio);
>   		goto again;
>   	}
>   	offset = swp_offset(folio->swap);
> -

Nit: unrelated change.

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

-- 
Cheers

David / dhildenb