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

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Message-ID: <CAJD7tkahj=QkNj1PBfzGRcDyrpE-n5Dbav7VVOVsAFR4vtx9LA@mail.gmail.com>
Date:   Thu, 16 Nov 2023 12:45:38 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Chris Li <chriscli@...gle.com>
Cc:     Zhongkun He <hezhongkun.hzk@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Nhat Pham <nphamcs@...il.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, Ying <ying.huang@...el.com>
Subject: Re: [PATCH] mm:zswap: fix zswap entry reclamation failure in two scenarios

On Thu, Nov 16, 2023 at 12:30 PM Chris Li <chriscli@...gle.com> wrote:
>
> On Thu, Nov 16, 2023 at 12:19 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > Not bypassing the swap slot cache, just make the callbacks to
> > invalidate the zswap entry, do memg uncharging, etc when the slot is
> > no longer used and is entering the swap slot cache (i.e. when
> > free_swap_slot() is called), instead of when draining the swap slot
> > cache (i.e. when swap_range_free() is called). For all parts of MM
> > outside of swap, the swap entry is freed when free_swap_slot() is
> > called. We don't free it immediately because of caching, but this
> > should be transparent to other parts of MM (e.g. zswap, memcg, etc).
>
> That will cancel the batching effect on the swap slot free, making the
> common case for  swapping  faults take longer to complete, righ?
> If I recall correctly, the uncharge is the expensive part of the swap
> slot free operation.
> I just want to figure out what we are trading off against. This is not
> one side wins all situations.

Interesting. Maybe we can just move the zswap_invalidate() call to
save memory early, and leave the memcg uncharge call to be batched?
IIUC we already do some version of this internally at Google.

>
>
> Chris