On Tue, Dec 10, 2013 at 03:03:39PM -0800, David Rientjes wrote: > On Tue, 10 Dec 2013, Mel Gorman wrote: > > > > If direct reclaim has failed to free memory, __GFP_NOFAIL allocations > > > can potentially loop forever in the page allocator. In this case, it's > > > better to give them the ability to access below watermarks so that they > > > may allocate similar to the same privilege given to GFP_ATOMIC > > > allocations. > > > > > > We're careful to ensure this is only done after direct reclaim has had > > > the chance to free memory, however. > > > > > > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx> > > > > The main problem with doing something like this is that it just smacks > > into the adjusted watermark if there are a number of __GFP_NOFAIL. Who > > was the user of __GFP_NOFAIL that was fixed by this patch? > > > > Nobody, it comes out of a memcg discussion where __GFP_NOFAIL were > recently given the ability to bypass charges to the root memcg when the > memcg has hit its limit since we disallow the oom killer to kill a process > (for the same reason that the vast majority of __GFP_NOFAIL users, those > that do GFP_NOFS | __GFP_NOFAIL, disallow the oom killer in the page > allocator). > > Without some other thread freeing memory, these allocations simply loop > forever. We probably don't want to reconsider the choice that prevents > calling the oom killer in !__GFP_FS contexts since it will allow > unnecessary oom killing when memory can actually be freed by another > thread. > > Since there are comments in both gfp.h and page_alloc.c that say no new > users will be added, it seems legitimate to ensure that the allocation > will at least have a chance of succeeding, but not the point of depleting > memory reserves entirely. > Which __GFP_NOFAIL on its own does not guarantee if they just smack into that barrier and cannot do anything. It changes the timing, not fixes the problem. > > There are enough bad users of __GFP_NOFAIL that I really question how > > good an idea it is to allow emergency reserves to be used when they are > > potentially leaked to other !__GFP_NOFAIL users via the slab allocator > > shortly afterwards. > > > > You could make the same argument for GFP_ATOMIC which can also allow > access to memory reserves. The critical difference being that GFP_ATOMIC callers typically can handle NULL being returned to them. GFP_ATOMIC storms may starve !GFP_ATOMIC requests but it does not cause the same types of problems that __GFP_NOFAIL using reserves would. -- Mel Gorman SUSE Labs -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>