linux-kernel - Re: [PATCH v3] mm/shmem, swap: fix race of truncate and swap entry split

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Message-ID: <CAKEwX=Omzgh92KHhaFi8-mnZ0myV1yi6XMTkT4FFsFPHFnueLQ@mail.gmail.com>
Date: Mon, 19 Jan 2026 13:36:14 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Hugh Dickins <hughd@...gle.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Chris Li <chrisl@...nel.org>, 
	Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>, linux-kernel@...r.kernel.org, 
	Kairui Song <kasong@...cent.com>, stable@...r.kernel.org
Subject: Re: [PATCH v3] mm/shmem, swap: fix race of truncate and swap entry split

On Mon, Jan 19, 2026 at 8:11 AM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.com>
>
> The helper for shmem swap freeing is not handling the order of swap
> entries correctly. It uses xa_cmpxchg_irq to erase the swap entry, but
> it gets the entry order before that using xa_get_order without lock
> protection, and it may get an outdated order value if the entry is split
> or changed in other ways after the xa_get_order and before the
> xa_cmpxchg_irq.
>
> And besides, the order could grow and be larger than expected, and cause
> truncation to erase data beyond the end border. For example, if the
> target entry and following entries are swapped in or freed, then a large
> folio was added in place and swapped out, using the same entry, the
> xa_cmpxchg_irq will still succeed, it's very unlikely to happen though.
>
> To fix that, open code the Xarray cmpxchg and put the order retrieval
> and value checking in the same critical section. Also, ensure the order
> won't exceed the end border, skip it if the entry goes across the
> border.
>
> Skipping large swap entries crosses the end border is safe here.
> Shmem truncate iterates the range twice, in the first iteration,
> find_lock_entries already filtered such entries, and shmem will
> swapin the entries that cross the end border and partially truncate the
> folio (split the folio or at least zero part of it). So in the second
> loop here, if we see a swap entry that crosses the end order, it must
> at least have its content erased already.
>
> I observed random swapoff hangs and kernel panics when stress testing
> ZSWAP with shmem. After applying this patch, all problems are gone.
>
> Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
> Cc: stable@...r.kernel.org
> Signed-off-by: Kairui Song <kasong@...cent.com>

Good catch.

>From the swap POV:

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