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Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries
From:   Ian Kent <raven@themaw.net>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andreas Dilger <adilger@dilger.ca>,
        Waiman Long <longman@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Luis Chamberlain <mcgrof@kernel.org>,
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        Iurii Zaikin <yzaikin@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Eric Biggers <ebiggers@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Eric Sandeen <sandeen@redhat.com>
Date:   Fri, 28 Feb 2020 12:16:39 +0800
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On Thu, 2020-02-27 at 19:34 -0800, Matthew Wilcox wrote:
> On Thu, Feb 27, 2020 at 05:55:43PM +0800, Ian Kent wrote:
> > Not all file systems even produce negative hashed dentries.
> > 
> > The most beneficial use of them is to improve performance of rapid
> > fire lookups for non-existent names. Longer lived negative hashed
> > dentries don't give much benefit at all unless they suddenly have
> > lots of hits and that would cost a single allocation on the first
> > lookup if the dentry ttl expired and the dentry discarded.
> > 
> > A ttl (say jiffies) set at appropriate times could be a better
> > choice all round, no sysctl values at all.
> 
> The canonical argument in favour of negative dentries is to improve
> application startup time as every application searches the library
> path
> for the same libraries.  Only they don't do that any more:
> 
> $ strace -e file cat /dev/null
> execve("/bin/cat", ["cat", "/dev/null"], 0x7ffd5f7ddda8 /* 44 vars
> */) = 0
> access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or
> directory)
> openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
> openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6",
> O_RDONLY|O_CLOEXEC) = 3
> openat(AT_FDCWD, "/usr/lib/locale/locale-archive",
> O_RDONLY|O_CLOEXEC) = 3
> openat(AT_FDCWD, "/dev/null", O_RDONLY) = 3
> 
> So, are they still useful?  Or should we, say, keep at most 100
> around?

Who knows how old apps will be on distros., ;)

But I don't think it matters.

The VFS will (should) work fine without a minimum negative hashed
dentry count (and hashed since unhashed negative dentries are
summarily executed on final dput()) and a ttl should keep frequently
used ones around long enough to satisfy this sort of thing should it
be needed.

Even the ttl value should be resilient to a large range of values,
just not so much very small ones.

Ian