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

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Date:   Sun, 19 Nov 2023 21:51:59 -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 9:41 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Chris Li <chrisl@...nel.org> writes:
>
> > On Sun, Nov 19, 2023 at 7:20 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Per my understanding, memcg uncharging happens in
>
> swapcache_free_entries()
>   swap_entry_free()
>     mem_cgroup_uncharge_swap()
>
> The swap entries are uncharged one-by-one, not

Yes. That matches my understanding as well.
I think I am using the term "batching" very loosely. My bad and thanks
for the clarification.

I am referring to the fact that in most cases, the free_swap_slot()
does not perform uncharge.
It is grouped together with other entries to uncharge together inside
swapcache_free_entries(). Yes, the uncharge itself is done by a for
loop page by page. No batching in the for loop. BTW, Not all uncharges
can be batched, because they can come from different memcg.

Chris