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: <CAKEwX=M6PpOQvqBK3hqfdVubdLE_hdnFHnsBMkxP3RJRmCZ3iw@mail.gmail.com>
Date: Mon, 15 Sep 2025 16:09:06 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, 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>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Ying Huang <ying.huang@...ux.alibaba.com>, Johannes Weiner <hannes@...xchg.org>, 
	David Hildenbrand <david@...hat.com>, 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 Wed, Sep 10, 2025 at 9:09 AM Kairui Song <ryncsn@...il.com> 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>

Apologies for the slow response. Swap code is dense, so it takes me
awhile to read each patch :(

Anyway, this LGTM!

Acked-by: Nhat Pham <nphamcs@...il.com>