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Date: Sat, 14 Jul 2018 19:35:16 +0200
From: Pavel Machek <pavel@ucw.cz>
To: Jan Kara <jack@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Waiman Long <longman@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
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	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Ingo Molnar <mingo@kernel.org>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Larry Woodman <lwoodman@redhat.com>,
	"Wangkai (Kevin,C)" <wangkai86@huawei.com>, linux-mm@kvack.org,
	Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries
Message-ID: <20180714173516.uumlhs4wgfgrlc32@devuan>
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> > Yes, "should be".  I could understand that the presence of huge
> > nunmbers of -ve dentries could result in undesirable reclaim of
> > pagecache, etc.  Triggering oom-killings is very bad, and presumably
> > has the same cause.
> > 
> > Before we go and add a large amount of code to do the shrinker's job
> > for it, we should get a full understanding of what's going wrong.  Is
> > it because the dentry_lru had a mixture of +ve and -ve dentries? 
> > Should we have a separate LRU for -ve dentries?  Are we appropriately
> > aging the various dentries?  etc.
> > 
> > It could be that tuning/fixing the current code will fix whatever
> > problems inspired this patchset.
> 
> What I think is contributing to the problems and could lead to reclaim
> oddities is the internal fragmentation of dentry slab cache. Dentries are
> relatively small, you get 21 per page on my system, so if trivial to
> reclaim negative dentries get mixed with a small amount of unreclaimable
> positive dentries, you can get a lot of pages in dentry slab cache that are
> unreclaimable.

Could we allocate -ve entries from separate slab?

-- 
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