linux-kernel - Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

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Date:   Thu, 14 Dec 2023 10:34:04 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     Nhat Pham <nphamcs@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chris Li <chriscli@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

[..]
>
> I think we shouldn't put these poorly compressed output into zswap,
> maybe it's better to early return in these cases when compress ratio
> < threshold ratio, which can be tune by the user?

We have something similar at Google, but because we use zswap without
a backing swapfile, we make those pages unevictable. For the upstream
code, the pages will go to a backing swapfile, which arguably violates
the LRU ordering, but may be the correct thing to do. There was a
recent upstream attempt to solidify storing those incompressible pages
in zswap in their uncompressed form to retain the LRU ordering.

If you want, feel free to start a discussion about this separately,
it's out of context for this patch series.

Thanks!