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Date: Mon, 16 Jul 2018 05:41:15 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Waiman Long <longman@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
	Jonathan Corbet <corbet@lwn.net>,
	"Luis R. Rodriguez" <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Jan Kara <jack@suse.cz>, Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Larry Woodman <lwoodman@redhat.com>,
	"Wangkai (Kevin,C)" <wangkai86@huawei.com>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries
Message-ID: <20180716124115.GA7072@bombadil.infradead.org>
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On Mon, Jul 16, 2018 at 11:09:01AM +0200, Michal Hocko wrote:
> On Fri 13-07-18 10:36:14, Dave Chinner wrote:
> [...]
> > By limiting the number of negative dentries in this case, internal
> > slab fragmentation is reduced such that reclaim cost never gets out
> > of control. While it appears to "fix" the symptoms, it doesn't
> > address the underlying problem. It is a partial solution at best but
> > at worst it's another opaque knob that nobody knows how or when to
> > tune.
> 
> Would it help to put all the negative dentries into its own slab cache?

Maybe the dcache should be more sensitive to its own needs.  In __d_alloc,
it could check whether there are a high proportion of negative dentries
and start recycling some existing negative dentries.