Re: [PATCH 2/9] mm, swap: always lock and check the swap cache folio before use

[Chris Li <chrisl@kernel.org>]

On Mon, Sep 1, 2025 at 11:18 AM Kairui Song <ryncsn@gmail.com> wrote:
> > So the missing readahead stats update behavior is the correct and
> > better behavior. I suggest you spit that out as a separate patch with
> > appropriate comments about it too. It is also easier to bisect the
> > commit if that kind of the subtle change which is considered safe
> > turns out causing a problem. Causing problem not happen very often but
> > it does happen before.
> >
>
> Hmm, after a second thought, maybe we should keep it as it is for now.
>
> I just realized moving the swap_update_readahead after folio_lock is
> more than just ensuring the folio is still valid. It will also cause
> every swapin to do a readahead update. Previously, only cache hit
> swapin will do a swap readahead update.
>
> I did some tests, and didn't see any measurable performance difference
> between putting it before / after the folio_lock. But changing it for
> no good reason seems not a good idea after all.
>
> So I think I'll keep it before the folio_lock. There is no evidence of
> which strategy is better, just keep the current behaviour.
>
> Calling swap_update_readahead even if the swap cache folio
> is already invalidated is not really harmful, the only thing it does
> that may effect the folio is the folio_test_clear_readahead call in
> it, and we have been doing for years with no problem. Calling
> swap_update_readahead for every folio might not be a good idea.

Thanks for the update. That is what I originally felt as well. It
caught me by surprise by this detour code path. I need to spend extra
time for the detour. I don't see it necessary to contribute to the
phase I goal of merging the swap table.

We can still add it later as a separate patch if we want.

Chris