linux-kernel - Re: [PATCH 2/2] zram: protect zram->stat race with init

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Date:	Fri, 23 Jan 2015 23:38:49 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 2/2] zram: protect zram->stat race with init_lock

On (01/23/15 14:58), Minchan Kim wrote:
> The zram->stat handling should be procted by init_lock.
> Otherwise, user could see stale value from the stat.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
> 
> I don't think it's stable material. The race is rare in real practice
> and this stale stat value read is not a critical.
> 
>  drivers/block/zram/zram_drv.c | 37 ++++++++++++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 0299d82275e7..53f176f590b0 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -48,8 +48,13 @@ static ssize_t name##_show(struct device *d,		\
>  				struct device_attribute *attr, char *b)	\
>  {									\

a side note: I wasn't Cc'd in that patchset and found out it only when it's
been merged. I'm not sure I understand, why it has been renamed from specific
zram_X_show to X_show. what gives?


can't help, catches my eye every time, that rename has broken the original
formatting:


diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 9250b3f..c567af5 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -44,7 +44,7 @@ static const char *default_compressor = "lzo";
 static unsigned int num_devices = 1;
 
 #define ZRAM_ATTR_RO(name)						\
-static ssize_t name##_show(struct device *d,		\
+static ssize_t name##_show(struct device *d,				\
 				struct device_attribute *attr, char *b)	\
 {									\
 	struct zram *zram = dev_to_zram(d);				\



I don't have any objections. but do we really want to wrap atomic ops in
semaphore? it is really such serious race?


	-ss

>  	struct zram *zram = dev_to_zram(d);				\
> -	return scnprintf(b, PAGE_SIZE, "%llu\n",			\
> -		(u64)atomic64_read(&zram->stats.name));			\
> +	u64 val = 0;							\
> +									\
> +	down_read(&zram->init_lock);					\
> +	if (init_done(zram))						\
> +		val = atomic64_read(&zram->stats.name);			\
> +	up_read(&zram->init_lock);					\
> +	return scnprintf(b, PAGE_SIZE, "%llu\n", val);			\
>  }									\
>  static DEVICE_ATTR_RO(name);
>  
> @@ -67,8 +72,14 @@ static ssize_t disksize_show(struct device *dev,
>  		struct device_attribute *attr, char *buf)
>  {
>  	struct zram *zram = dev_to_zram(dev);
> +	u64 val = 0;
> +
> +	down_read(&zram->init_lock);
> +	if (init_done(zram))
> +		val = zram->disksize;
> +	up_read(&zram->init_lock);
>  
> -	return scnprintf(buf, PAGE_SIZE, "%llu\n", zram->disksize);
> +	return scnprintf(buf, PAGE_SIZE, "%llu\n", val);
>  }
>  
>  static ssize_t initstate_show(struct device *dev,
> @@ -88,9 +99,14 @@ static ssize_t orig_data_size_show(struct device *dev,
>  		struct device_attribute *attr, char *buf)
>  {
>  	struct zram *zram = dev_to_zram(dev);
> +	u64 val = 0;
> +
> +	down_read(&zram->init_lock);
> +	if (init_done(zram))
> +		val = atomic64_read(&zram->stats.pages_stored) << PAGE_SHIFT;
> +	up_read(&zram->init_lock);
>  
> -	return scnprintf(buf, PAGE_SIZE, "%llu\n",
> -		(u64)(atomic64_read(&zram->stats.pages_stored)) << PAGE_SHIFT);
> +	return scnprintf(buf, PAGE_SIZE, "%llu\n", val);
>  }
>  
>  static ssize_t mem_used_total_show(struct device *dev,
> @@ -957,10 +973,6 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
>  	struct bio_vec bv;
>  
>  	zram = bdev->bd_disk->private_data;
> -	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
> -		atomic64_inc(&zram->stats.invalid_io);
> -		return -EINVAL;
> -	}
>  
>  	down_read(&zram->init_lock);
>  	if (unlikely(!init_done(zram))) {
> @@ -968,6 +980,13 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
>  		goto out_unlock;
>  	}
>  
> +	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
> +		atomic64_inc(&zram->stats.invalid_io);
> +		err = -EINVAL;
> +		goto out_unlock;
> +	}
> +
> +
>  	index = sector >> SECTORS_PER_PAGE_SHIFT;
>  	offset = sector & (SECTORS_PER_PAGE - 1) << SECTOR_SHIFT;
>  
> -- 
> 1.9.1
> 
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