linux-kernel - Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls

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Date:	Wed, 7 Jul 2010 13:37:22 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Andreas Dilger <andreas.dilger@...cle.com>
Cc:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Neil Brown <neilb@...e.de>, hch@...radead.org,
	viro@...iv.linux.org.uk, adilger@....COM, corbet@....net,
	serue@...ibm.com, hooanon05@...oo.co.jp,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle
 syscalls

On Wed, Jul 07, 2010 at 11:02:47AM -0600, Andreas Dilger wrote:
> On 2010-07-07, at 09:05, J. Bruce Fields wrote:
> > On Wed, Jul 07, 2010 at 01:40:53AM -0600, Andreas Dilger wrote:
> >> On 2010-07-06, at 11:09, Aneesh Kumar K. V wrote:
> >>> Since we know that system wide file handle should include a file system
> >>> identifier and a file identifier my plan was to retrieve both in the
> >>> same syscall.
> >> 
> >> Won't having it be in a separate system call be racy w.r.t. doing the pathname lookup twice?
> > 
> > It'll be rare that a server will want to *just* get a filehandle;
> > normally it will at least want to get some attributes at the same time.
> > So I think it will always need to open the file first and then do the
> > rest of the operations on the returned filehandle.
> 
> I think you are assuming too much about the use of the file handle.  What I'm interested in is not a userspace file server, but rather a more efficient way to have 10000's to millions of clients to be able to open the same regular file, without having to do full path traversal for each one.

Understood.  I don't understand how that case decides the question of
whether to use a separate system call for the uuid or not?

Those millions of clients are doing the filehandle-to-file-descriptor
mapping, not the reverse.

And you may not need any uuid at all since the clients probably all have
some way of knowing which shared filesystem they want to work with.

> >>> That still leaves the problem that there isn't always an
> >>> underlying block device, and/or when there is it doesn't always
> >>> uniquely specify the filesystem.
> >> 
> >> And for this reason we would need this as a syscall right ?
> > 
> > That's the only solution I see.  (Or use an xattr?)
> 
> Or... return the UUID as part of the file handle in the first place.
> That avoids races, avoids adding more syscalls that have to be called
> for each file handle,

I don't hate the idea.  A uuid lookup seems like a useful thing, though,
and maybe less expensive, so it seems weird if the only way to get the
uuid is by looking up the filehandle as well.

> or IMNSHO the worst proposal that requires
> applications to parse a text file in some obscure path for each file
> handle (requiring a stat() to find the major/minor device of the file,
> walking through /proc or /sys, and other nastiness).

OK.

--b.
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