linux-kernel - Re: [PATCH 1/3] zswap: disable changing params if init fails

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Date:   Tue, 24 Jan 2017 13:24:41 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Dan Streetman <ddstreet@...e.org>
Cc:     Linux-MM <linux-mm@...ck.org>, Seth Jennings <sjenning@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Dan Streetman <dan.streetman@...onical.com>
Subject: Re: [PATCH 1/3] zswap: disable changing params if init fails

On Tue, 24 Jan 2017 15:02:57 -0500 Dan Streetman <ddstreet@...e.org> wrote:

> Add zswap_init_failed bool that prevents changing any of the module
> params, if init_zswap() fails, and set zswap_enabled to false.  Change
> 'enabled' param to a callback, and check zswap_init_failed before
> allowing any change to 'enabled', 'zpool', or 'compressor' params.
> 
> Any driver that is built-in to the kernel will not be unloaded if its
> init function returns error, and its module params remain accessible for
> users to change via sysfs.  Since zswap uses param callbacks, which
> assume that zswap has been initialized, changing the zswap params after
> a failed initialization will result in WARNING due to the param callbacks
> expecting a pool to already exist.  This prevents that by immediately
> exiting any of the param callbacks if initialization failed.
> 
> This was reported here:
> https://marc.info/?l=linux-mm&m=147004228125528&w=4

I added Marcin's reportde-by to the changelog.

> And fixes this WARNING:
> [  429.723476] WARNING: CPU: 0 PID: 5140 at mm/zswap.c:503
> __zswap_pool_current+0x56/0x60
> 
> Fixes: 90b0fc26d5db ("zswap: change zpool/compressor at runtime")
> Cc: stable@...r.kernel.org

Is this really serious enough to justify a -stable backport?  It's just
a bit of extra noise associated with an initialization problem which
the user will be fixing anyway.