On Tue 14-04-15 10:11:18, Dave Chinner wrote: > On Mon, Apr 13, 2015 at 02:46:14PM +0200, Michal Hocko wrote: > > [Sorry for a late reply] > > > > On Tue 07-04-15 10:18:22, Johannes Weiner wrote: > > > On Wed, Apr 01, 2015 at 05:19:20PM +0200, Michal Hocko wrote: > > > My question here would be: are there any NOFS allocations that *don't* > > > want this behavior? Does it even make sense to require this separate > > > annotation or should we just make it the default? > > > > > > The argument here was always that NOFS allocations are very limited in > > > their reclaim powers and will trigger OOM prematurely. However, the > > > way we limit dirty memory these days forces most cache to be clean at > > > all times, and direct reclaim in general hasn't been allowed to issue > > > page writeback for quite some time. So these days, NOFS reclaim isn't > > > really weaker than regular direct reclaim. > > > > What about [di]cache and some others fs specific shrinkers (and heavy > > metadata loads)? > > We don't do direct reclaim for fs shrinkers in GFP_NOFS context, > either. Yeah but we invoke fs shrinkers for the _regular_ direct reclaim (with __GFP_FS), which was the point I've tried to make here. > *HOWEVER* > > The shrinker reclaim we can not execute is deferred to the next > context that can do the reclaim, which is usually kswapd. So the > reclaim gets done according to the GFP_NOFS memory pressure that is > occurring, it is just done in a different context... Right, deferring to kswapd is the reason why I think the direct reclaim shouldn't invoke OOM killer in this context because that would be premature - as kswapd still can make some progress. Sorry for not being more clear. > > > The only exception is that > > > it might block writeback, so we'd go OOM if the only reclaimables left > > > were dirty pages against that filesystem. That should be acceptable. > > > > OOM killer is hardly acceptable by most users I've heard from. OOM > > killer is the _last_ resort and if the allocation is restricted then > > we shouldn't use the big hammer. The allocator might use __GFP_HIGH to > > get access to memory reserves if it can fail or __GFP_NOFAIL if it > > cannot. With your patches the NOFAIL case would get an access to memory > > reserves as well. So I do not really see a reason to change GFP_NOFS vs. > > OOM killer semantic. > > So, really, what we want is something like: > > #define __GFP_USE_LOWMEM_RESERVE __GFP_HIGH > > So that it documents the code that is using it effectively and we > can find them easily with cscope/grep? I wouldn't be opposed. To be honest I was never fond of __GFP_HIGH. The naming is counterintuitive. So I would rather go with renaminag it. We do not have that many users in the tree. git grep "GFP_HIGH\>" | wc -l 40 -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
