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

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Date:   Sun, 19 Nov 2023 21:31:30 -0800
From:   Chris Li <chrisl@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        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>
Subject: Re: [PATCH] mm:zswap: fix zswap entry reclamation failure in two scenarios

On Sun, Nov 19, 2023 at 7:20 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Chris Li <chriscli@...gle.com> writes:
>
> > On Thu, Nov 16, 2023 at 12:19 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > 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.
>
> Per my understanding, we don't batch memcg uncharging in
> swap_entry_free() now.  Although it's possible and may improve
> performance.

swap_entry_free() does not do batching, it is at the caller level.

I just checked. The batching is done in free_swap_slot() is still
using swap slot cache and batching.
It uses swapcache_free_entries() to batch free the swap_slots. That is
where the uncharge happens per my understanding.

Chris