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

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Message-ID: <CAGsJ_4ygTv1tCJeuF43NhRR4E0kiMLpk6i8c+UHoUMt6LXykww@mail.gmail.com>
Date: Wed, 20 Aug 2025 13:13:13 +1200
From: Barry Song <21cnbao@...il.com>
To: SeongJae Park <sj@...nel.org>, Herbert Xu <herbert@...dor.apana.org.au>
Cc: 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

[...]
> +
> +       /*
> +        * If a page cannot be compressed into a size smaller than PAGE_SIZE,
> +        * save the content as is without a compression, to keep the LRU order
> +        * of writebacks.  If writeback is disabled, reject the page since it
> +        * only adds metadata overhead.  swap_writeout() will put the page back
> +        * to the active LRU list in the case.
> +        */
> +       if (comp_ret || !dlen) {
> +               zswap_crypto_compress_fail++;
> +               dlen = PAGE_SIZE;
> +       }

I’m not entirely sure about this. As long as we pass 2*PAGE_SIZE as
dst_buf, any error returned by crypto drivers should indicate a bug in
the driver that needs to be fixed.

There have also been attempts to unify crypto drivers so they consistently
return -ENOSPC when the destination buffer would overflow. If that has
been achieved, we might be able to reduce the buffer from 2*PAGE_SIZE to
just PAGE_SIZE in zswap. There was a long discussion on this here:
https://lore.kernel.org/linux-mm/Z7dnPh4tPxLO1UEo@google.com/

I’m not sure of the current status — do all crypto drivers now return a
consistent -ENOSPC when the compressed size exceeds PAGE_SIZE? From
what I recall during that discussion, most drivers already behaved this
way, but Sergey Senozhatsky pointed out one or two exceptions.

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?

Thanks
Barry