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

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Message-ID: <CAKEwX=Py+yvxtR5zt-1DtskhGWWHkRP_h8kneEHSrcQ947=m9Q@mail.gmail.com>
Date: Wed, 30 Jul 2025 17:22:51 -0700
From: Nhat Pham <nphamcs@...il.com>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Chengming Zhou <chengming.zhou@...ux.dev>, 
	Johannes Weiner <hannes@...xchg.org>, Takero Funaki <flintglass@...il.com>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, kernel-team@...a.com, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [RFC PATCH] mm/zswap: store compression failed page as-is

On Wed, Jul 30, 2025 at 5:21 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Wed, Jul 30, 2025 at 4:41 PM SeongJae Park <sj@...nel.org> wrote:
>
> This might not be safe.
>
> It's conceivable that in zswap_compress(), some compression algorithm
> "successfully" compresses a page to the same size (comp_ret == 0). We
> hand that to zsmalloc, which happily stores the page.
>
> When we "decompress" the page again, we will attempt to
> memcpy_to_folio from a bogus address (the handle from zsmalloc).
>
> So, in zswap_compress, you have to treat both comp_ret == 0 and dlen
> == PAGE_SIZE as "compression failure".

Meant to say comp_ret != 0 here... sorry for the confusion...