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

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Date:	Wed, 5 Jun 2013 15:29:35 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	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 Wed, Jun 05, 2013 at 12:06:01AM +0800, 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.)

I think it's possible if the permission is allowed so we should take care
of that. But I'd like to clear your comment about "invalid memory access".

As I read the code, one of the problem we can encounter by race between
zram_bvec_read and zram_slot_free_notify is BUG_ON(!handle) on zs_map_object
or pr_err("Decompression failed xxxx) on zram_decompress_page.
Otherwise, it would be able to access different swap block with
user request's one.

Could you please expand your vague "invalid memory access"?


> 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().

OK.

And please add the comment struct zram->lock feild, too.

Thanks.

> 
> 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);
>  }
>  
> -- 
> 1.8.1.2
> 
> --
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-- 
Kind regards,
Minchan Kim
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