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From:   Chris Li <chriscli@google.com>
Date:   Thu, 16 Nov 2023 12:30:20 -0800
Message-ID: <CAF8kJuOC30feLGs0bNHOxMjSZ3uqF1y7eUdJ4p-w-myP8c1cFg@mail.gmail.com>
Subject: Re: [PATCH] mm:zswap: fix zswap entry reclamation failure in two scenarios
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Zhongkun He <hezhongkun.hzk@bytedance.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Nhat Pham <nphamcs@gmail.com>,
        Seth Jennings <sjenning@redhat.com>,
        Dan Streetman <ddstreet@ieee.org>,
        Vitaly Wool <vitaly.wool@konsulko.com>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, Ying <ying.huang@intel.com>
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On Thu, Nov 16, 2023 at 12:19=E2=80=AFPM Yosry Ahmed <yosryahmed@google.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.


Chris