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

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Message-ID: <CAF8kJuPPsLzWu8+xm2A_UPHMBhb7OTjJNErM1Kp3hPmvHXNDUQ@mail.gmail.com>
Date: Tue, 19 Aug 2025 22:20:45 -0700
From: Chris Li <chrisl@...nel.org>
To: Herbert Xu <herbert@...dor.apana.org.au>
Cc: Barry Song <21cnbao@...il.com>, 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>, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v4] mm/zswap: store <PAGE_SIZE compression failed page as-is

On Tue, Aug 19, 2025 at 10:11 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Tue, Aug 19, 2025 at 10:07:57PM -0700, Chris Li wrote:
> >
> > But if that error case can never happen, we should convert the crypto
> > compression from returning error number to bool instead. It does not
> > make sense to return a free form error code and also demand the caller
> > does not check on it. If you insist that the caller should check
> > return value like a boolean, just let the API return a boolean.
>
> No we need the error value for other things, such as asynchronous
> return -EINPROGRESS.

In that case it is fair game for the caller to check for the error
other than -ENOSPC. If the return value is -EINPROGRESS during
synchronous crypto compression, it is a bug. We are not encouraged to
use WARN_ON, then having an error count to check that something truly
unlikely but possible to happen is better than folding it into
incompressible and pretend the error never happened. At least we can
know such an error case happened.

Chris