RE: [PATCH] staging: zcache: remove zcache_direct_reclaim_lock

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Seth Jennings [mailto:sjenning@linux.vnet.ibm.com]
> Subject: [PATCH] staging: zcache: remove zcache_direct_reclaim_lock
> 
> zcache_do_preload() currently does a spin_trylock() on the
> zcache_direct_reclaim_lock. Holding this lock intends to prevent
> shrink_zcache_memory() from evicting zbud pages as a result
> of a preload.
>
> However, it also prevents two threads from
> executing zcache_do_preload() at the same time.  The first
> thread will obtain the lock and the second thread's spin_trylock()
> will fail (an aborted preload) causing the page to be either lost
> (cleancache) or pushed out to the swap device (frontswap). It
> also doesn't ensure that the call to shrink_zcache_memory() is
> on the same thread as the call to zcache_do_preload().

Yes, this looks to be leftover code from early in kztmem/zcache
development.  Good analysis.
 
> Additional, there is no need for this mechanism because all
> zcache_do_preload() calls that come down from cleancache already
> have PF_MEMALLOC set in the process flags which prevents
> direct reclaim in the memory manager. If the zcache_do_preload()

Might it be worthwhile to add a BUG/ASSERT for the presence
of PF_MEMALLOC, or at least a comment in the code?

> call is done from the frontswap path, we _want_ reclaim to be
> done (which it isn't right now).
> 
> This patch removes the zcache_direct_reclaim_lock and related
> statistics in zcache.
> 
> Based on v3.1-rc8
> 
> Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
> Reviewed-by: Dave Hansen <dave@linux.vnet.ibm.com>

With added code/comment per above...
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>