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Date: Mon, 2 Jul 2018 15:21:05 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Waiman Long <longman@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Jan Kara <jack@suse.cz>, Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Ingo Molnar <mingo@kernel.org>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Larry Woodman <lwoodman@redhat.com>,
	James Bottomley <James.Bottomley@hansenpartnership.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: <20180702222105.GA2438@bombadil.infradead.org>
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On Mon, Jul 02, 2018 at 02:18:11PM -0700, Andrew Morton wrote:
> In the [5/6] changelog it is mentioned that a large number of -ve
> dentries can lead to oom-killings.  This sounds bad - -ve dentries
> should be trivially reclaimable and we shouldn't be oom-killing in such
> a situation.
> 
> Dumb question: do we know that negative dentries are actually
> worthwhile?  Has anyone checked in the past couple of decades?  Perhaps
> our lookups are so whizzy nowadays that we don't need them?

I can't believe that's true.  Have you looked at strace of a typical
program startup recently?

$ strace -o ls.out ls 
$ grep -c ENOENT ls.out 
10

There's a few duplicates in there (6 accesses to /etc/ld.so.nohwcap), so
we definitely want those negative entries.