On 10/12/2011 03:39 PM, Dan Magenheimer wrote: >> From: Seth Jennings [mailto:sjenning@xxxxxxxxxxxxxxxxxx] >> 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? I was mistaken in my commit comments. Not all cleancache calls have PF_MEMALLOC set. One exception is calls from the cgroup code paths. However, there isn't a way for the code to loop back on itself. Regardless of whether or not PF_MEMALLOC is set coming into the preload, the call path only goes one way: zcache_do_preload() kmem_cache_alloc() possibly reclaim and call to shrink_zcache_memory() zbud_evict_pages() Nothing done in zbud_evict_pages() can result in a call back to zcache_do_preload(). So there isn't a threat of recursion. NOW, if the logic your are trying to implement is: "Don't kick out zbud pages as the result of preload allocations" then that's a different story. If the preload is called with PF_MEMALLOC set, then the shrinker will not be run during a kmem_cache_alloc(). However if the preload is called with PF_MEMALLOC being set then there is a chance that some zbud pages might be reclaimed as a result. BUT, I'm not convinced that is a bad thing. > >> 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@xxxxxxxxxxxxxxxxxx> >> Reviewed-by: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx> > > With added code/comment per above... > Acked-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>