linux-kernel - Re: [PATCH 15/18] fs: introduce a per-cpu last

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Date:	Fri, 08 Oct 2010 12:20:19 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/18] fs: introduce a per-cpu last_ino allocator

Le vendredi 08 octobre 2010 à 06:03 -0400, Christoph Hellwig a écrit :
> On Fri, Oct 08, 2010 at 10:56:58AM +0100, Al Viro wrote:
> > FWIW, that one is begging to be split; what I mean is that there are
> > two classes of callers; ones that will set i_ino themselves anyway
> > and ones that really want i_ino invented.  Two functions?
> 
> There's no reason to add i_ino before adding it to the per-sb list,
> we don't do so either for inodes acquired via iget.  The fix is simply
> to stop assigning i_ino in new_inode and call the helper to get it in
> the place that need it after the call to new_inode.  Later we can
> even move to a lazy assignment scheme where needed.  I'd also really
> like to get a grip on why the simple counters if fine for some
> filesystems while we need iunique() for others.

If iunique() was scalable, sockets could use it, so that we can have
hard guarantee two sockets on machine dont have same inum.

A reasonable compromise here is to use a simple and scalable allocator,
and take the risk two sockets have same inum.

While it might break some applications playing fstats() games, on
sockets, current schem is vastly faster.

I worked with machines with millions of opened socket concurrently,
iunique() was not an option, and application didnt care of possible inum
clash.

For disk files, inum _must_ be unique per fs, for sockets, its only if
you want strict compliance to some standards.

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