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

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Date:   Mon, 20 Nov 2023 13:39:35 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Chris Li <chrisl@...nel.org>
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

Chris Li <chrisl@...nel.org> writes:

> 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.

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

acquire lock in memcg
uncharge all entries
release lock in memcg

--
Best Regards,
Huang, Ying