Re: [PATCH] mm/zswap: use workqueue to destroy pool

[Sergey Senozhatsky]

On (04/26/16 17:08), Dan Streetman wrote:
> Add a work_struct to struct zswap_pool, and change __zswap_pool_empty
> to use the workqueue instead of using call_rcu().
> 
> When zswap destroys a pool no longer in use, it uses call_rcu() to
> perform the destruction/freeing.  Since that executes in softirq
> context, it must not sleep.  However, actually destroying the pool
> involves freeing the per-cpu compressors (which requires locking the
> cpu_add_remove_lock mutex) and freeing the zpool, for which the
> implementation may sleep (e.g. zsmalloc calls kmem_cache_destroy,
> which locks the slab_mutex).  So if either mutex is currently taken,
> or any other part of the compressor or zpool implementation sleeps, it
> will result in a BUG().
> 
> It's not easy to reproduce this when changing zswap's params normally.
> In testing with a loaded system, this does not fail:
> 
> $ cd /sys/module/zswap/parameters
> $ echo lz4 > compressor ; echo zsmalloc > zpool
> 
> nor does this:
> 
> $ while true ; do
> > echo lzo > compressor ; echo zbud > zpool
> > sleep 1
> > echo lz4 > compressor ; echo zsmalloc > zpool
> > sleep 1
> > done
> 
> although it's still possible either of those might fail, depending on
> whether anything else besides zswap has locked the mutexes.
> 
> However, changing a parameter with no delay immediately causes the
> schedule while atomic BUG:
> 
> $ while true ; do
> > echo lzo > compressor ; echo lz4 > compressor
> > done
> 
> This is essentially the same as Yu Zhao's proposed patch to zsmalloc,
> but moved to zswap, to cover compressor and zpool freeing.
> 
> Fixes: f1c54846ee45 ("zswap: dynamic pool creation")
> Reported-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> Signed-off-by: Dan Streetman <ddstreet@xxxxxxxx>
> Cc: Dan Streetman <dan.streetman@xxxxxxxxxxxxx>

Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>

	-ss