linux-kernel - Re: [PATCH v4] mm/zswap: store <PAGE_SIZE compression failed page as-is

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Message-ID: <CAGsJ_4z6YvQULrEmNjFjLNrJ4RK6w0+d9uF2-7v06gOYirrYRw@mail.gmail.com>
Date: Wed, 20 Aug 2025 13:34:01 +1200
From: Barry Song <21cnbao@...il.com>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: SeongJae Park <sj@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Nhat Pham <nphamcs@...il.com>, Yosry Ahmed <yosry.ahmed@...ux.dev>, kernel-team@...a.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Takero Funaki <flintglass@...il.com>, David Hildenbrand <david@...hat.com>, Baoquan He <bhe@...hat.com>, 
	Chris Li <chrisl@...nel.org>, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v4] mm/zswap: store <PAGE_SIZE compression failed page as-is

On Wed, Aug 20, 2025 at 1:21 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Wed, Aug 20, 2025 at 01:13:13PM +1200, Barry Song wrote:
> >
> > Let’s sync with Herbert: have we reached the stage where all drivers
> > reliably return -ENOSPC when dst_buf is PAGE_SIZE but the compressed
> > size would exceed it?
>
> It doesn't matter.  Software compression should never fail, and
> if it does fail due to a bug, that's not something that the user
> of the API should worry about.
>
> Hardware compression should always fall back to software compression
> in case of failure.
>
> IOW all compression errors should be treated as incompressible
> data.

That is why I think it makes little sense to add a counter
zswap_crypto_compress_fail, since zswap already passes 2*PAGE_SIZE as
dst_buf, which is large enough to hold even incompressible data. Adding
a counter that will always stay at zero seems meaningless.

We might want to revisit the old thread to check whether it is now safe for us
to move to PAGE_SIZE in zswap now.

Thanks
Barry