linux-kernel - Re: [PATCH v2 03/10] zram: use zram->lock to protect zram_free

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Message-ID: <51AF1242.809@redhat.com>
Date:	Wed, 05 Jun 2013 12:26:10 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Jiang Liu <liuj97@...il.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Yijing Wang <wangyijing@...wei.com>,
	Jiang Liu <jiang.liu@...wei.com>, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/10] zram: use zram->lock to protect zram_free_page()
 in swap free notify path

On 06/04/2013 06:06 PM, Jiang Liu wrote:
> zram_free_page() is protected by down_write(&zram->lock) when called by
> zram_bvec_write(), but there's no such protection when called by
> zram_slot_free_notify(), which may cause wrong states to zram object.
> 
> There are two possible consequences of this issue:
> 1) It may cause invalid memory access if we read from a zram device used
>    as swap device. (Not sure whether it's legal to make read/write
>    requests to a zram device used as swap device.)

It's never a good idea to mess with a swap device in use, but AFAICT
nothing prevents it (save for file permissions and admin sanity).

Jerome

> 2) It may cause some fields (bad_compress, good_compress, pages_stored)
>    in zram->stats wrong if the swap layer makes concurrently call to
>    zram_slot_free_notify().
> 
> So enhance zram_slot_free_notify() to acquire writer lock on zram->lock
> before calling zram_free_page().
> 
> Signed-off-by: Jiang Liu <jiang.liu@...wei.com>
> Cc: stable@...r.kernel.org
> ---
>  drivers/staging/zram/zram_drv.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
> index 5a2f20b..847d207 100644
> --- a/drivers/staging/zram/zram_drv.c
> +++ b/drivers/staging/zram/zram_drv.c
> @@ -582,7 +582,9 @@ static void zram_slot_free_notify(struct block_device *bdev,
>  	struct zram *zram;
>  
>  	zram = bdev->bd_disk->private_data;
> +	down_write(&zram->lock);
>  	zram_free_page(zram, index);
> +	up_write(&zram->lock);
>  	zram_stat64_inc(zram, &zram->stats.notify_free);
>  }
>  
> 

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