Re: [PATCH v2 03/10] zram: use zram->lock to protectzram_free_page() in swap free notify path
From: Minchan Kim
Date: Wed Jun 05 2013 - 02:29:56 EST
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
> So enhance zram_slot_free_notify() to acquire writer lock on zram->lock
> before calling zram_free_page().
And please add the comment struct zram->lock feild, too.
> Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> 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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