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From: SeongJae Park <sj@kernel.org>
To: Nhat Pham <nphamcs@gmail.com>
Cc: SeongJae Park <sj@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Chengming Zhou <chengming.zhou@linux.dev>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Takero Funaki <flintglass@gmail.com>,
	Yosry Ahmed <yosry.ahmed@linux.dev>,
	kernel-team@meta.com,
	linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [RFC PATCH] mm/zswap: store compression failed page as-is
Date: Thu, 31 Jul 2025 09:43:56 -0700
Message-Id: <20250731164356.15198-1-sj@kernel.org>
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On Wed, 30 Jul 2025 17:22:51 -0700 Nhat Pham <nphamcs@gmail.com> wrote:

> On Wed, Jul 30, 2025 at 5:21 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Wed, Jul 30, 2025 at 4:41 PM SeongJae Park <sj@kernel.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...

No worry, thank you for clarifying!


Thanks,
SJ