linux-kernel - Re: [External] Re: [PATCH] mm:zswap: fix zswap entry reclamation failure in two scenarios

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Date:   Wed, 15 Nov 2023 20:09:49 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     贺中坤 <hezhongkun.hzk@...edance.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, nphamcs@...il.com,
        sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Ying <ying.huang@...el.com>
Subject: Re: [External] Re: [PATCH] mm:zswap: fix zswap entry reclamation
 failure in two scenarios

On Wed, Nov 15, 2023 at 7:33 PM 贺中坤 <hezhongkun.hzk@...edance.com> wrote:
>
> On Thu, Nov 16, 2023 at 4:13 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > I think we may need to try to lock the folio. Otherwise we may race
> > with reclaim reading the dirty bit before we set it.
> >
> > Taking a step back, this seems like we are going behind exclusive
> > loads. We "should" keep the page in zswap as exclusive loads are
> > disabled and the page is not yet invalidated from zswap (the swap
> > entry is still in use). What you are trying to do here is sneakily
> > drop the page from zswap as if we wrote it back, but we didn't.
>
> If  we want to reclaim the cached zswap_entry, Writing back might
> be the easy way.
>
> > We just know that it was already loaded from zswap. We are essentially
> > making the previous load exclusive retroactively.
> >
> > Is there a reason why exclusive loads cannot be enabled to achieve the
> > same result in the (arguably) correct way?
> >
>
> zswap_exclusive_loads is not enabled by default, so the shrink_worker
> may fail if there are many cached zswap_entries on the zswap_pool->lru.


It can be enabled at runtime, or enabled by default by using
CONFIG_ZSWAP_EXCLUSIVE_LOADS_DEFAULT_ON.

>
>
> Is it possible to make zswap_exclusive_loads the only way in zswap_load?
> It only makes sense when the page is read and no longer dirty.
> If the page is read frequently, it should stay in cache rather than zswap.
> The benefit of doing this is very small, two copies of the same page
> in memory.


The reason I added it behind runtime and config knobs is to preserve
the existing behavior in case someone depends on it. At Google, we
have been using exclusive loads for a long time. If other users of
zswap agree to make this the default behavior or make it the only way
to do zswap loads I don't have a problem with it.

>
>
> Thanks.