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

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Date:   Fri, 17 Nov 2023 15:47:49 -0800
From:   Chris Li <chrisl@...nel.org>
To:     Zhongkun He <hezhongkun.hzk@...edance.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.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: [External] Re: [PATCH] mm:zswap: fix zswap entry reclamation
 failure in two scenarios

On Fri, Nov 17, 2023 at 1:56 AM Zhongkun He
<hezhongkun.hzk@...edance.com> wrote:
> Hi Chris, thanks for your feedback.  I have the same concerns,
> maybe we should just move the zswap_invalidate() out of batches,
> as Yosry mentioned above.

As I replied in the previous email, I just want to understand the
other side effects of the change better.

To me, this patching is actually freeing the memory that does not
require actual page IO write from zswap. Which means the memory is
from some kind of cache. It would be interesting if we can not
complicate the write back path further. Instead, we can drop those
memories from the different cache if needed. I assume those caches are
doing something useful in the common case. If not, we should have a
patch to remove these caches instead.  Not sure how big a mess it will
be to implement separate the write and drop caches.

While you are here, I have some questions for you.

Can you help me understand how much memory you can free from this
patch? For example, are we talking about a few pages or a few GB?

Where does the freed memory come from?
If the memory comes from zswap entry struct. Due to the slab allocator
fragmentation. It would take a lot of zswap entries to have meaningful
memory reclaimed from the slab allocator.

If the memory comes from the swap cached pages, that would be much
more meaningful. But that is not what this patch is doing, right?

Chris