linux-kernel - Re: [PATCH v4 3/3] zram: support BDI_CAP_STABLE

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Date:   Thu, 22 Dec 2016 10:31:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Takashi Iwai <tiwai@...e.de>,
        Hyeoncheol Lee <cheol.lee@....com>, yjay.kim@....com,
        Sangseok Lee <sangseok.lee@....com>,
        Hugh Dickins <hughd@...gle.com>,
        "[4.7+]" <stable@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] zram: support BDI_CAP_STABLE_WRITES

On (12/22/16 09:36), Minchan Kim wrote:
> zram has used per-cpu stream feature from v4.7.
> It aims for increasing cache hit ratio of scratch buffer for
> compressing. Downside of that approach is that zram should ask
> memory space for compressed page in per-cpu context which requires
> stricted gfp flag which could be failed. If so, it retries to
> allocate memory space out of per-cpu context so it could get memory
> this time and compress the data again, copies it to the memory space.
> 
> In this scenario, zram assumes the data should never be changed
> but it is not true without stable page support. So, If the data is
> changed under us, zram can make buffer overrun so that zsmalloc
> free object chain is broken so system goes crash like below
> https://bugzilla.suse.com/show_bug.cgi?id=997574
> 
> This patch adds BDI_CAP_STABLE_WRITES to zram for declaring
> "I am block device needing *stable write*".
> 
> Fixes: da9556a2367c ("zram: user per-cpu compression streams")
> Cc: <stable@...r.kernel.org> [4.7+]
> Signed-off-by: Minchan Kim <minchan@...nel.org>

Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

	-ss