Re: [PATCH] staging: zcache: fix cleancache race condition withshrinker

[Minchan Kim]

On Wed, Aug 29, 2012 at 04:58:45PM -0500, Seth Jennings wrote:
> This patch fixes a race condition that results in memory
> corruption when using cleancache.
> 
> The race exists between the zcache shrinker handler,
> shrink_zcache_memory() and cleancache_get_page().
> 
> In most cases, the shrinker will both evict a zbpg
> from its buddy list and flush it from tmem before a
> cleancache_get_page() occurs on that page. A subsequent
> cleancache_get_page() will fail in the tmem layer.
> 
> In the rare case that two occur together and the
> cleancache_get_page() path gets through the tmem
> layer before the shrinker path can flush tmem,
> zbud_decompress() does a check to see if the zbpg is a
> "zombie", i.e. not on a buddy list, which means the shrinker
> is in the process of reclaiming it. If the zbpg is a zombie,
> zbud_decompress() returns -EINVAL.
> 
> However, this return code is being ignored by the caller,
> zcache_pampd_get_data_and_free(), which results in the
> caller of cleancache_get_page() thinking that the page has
> been properly retrieved when it has not.
> 
> This patch modifies zcache_pampd_get_data_and_free() to
> convey the failure up the stack so that the caller of
> cleancache_get_page() knows the page retrieval failed.
> 
> ---
> Based on v3.6-rc3.
> 
> This needs to be applied to stable trees as well.
> zcache-main.c was named zcache.c before v3.1, so
> I'm not sure how you want to handle trees earlier
> than that.
> 
> Signed-off-by: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>

Thanks!
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