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

Nhat Pham
2025-09-15T23:09:21Z

On Wed, Sep 10, 2025 at 9:09 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> From: Kairui Song <kasong@tencent.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@tencent.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@gmail.com>