linux-kernel - Re: [PATCH -V7 4/9] vfs: Add open by file handle support

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Message-ID: <87pr101f5d.fsf@linux.vnet.ibm.com>
Date:	Thu, 13 May 2010 12:07:02 +0530
From:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>
Cc:	hch@...radead.org, viro@...iv.linux.org.uk, adilger@....com,
	corbet@....net, serue@...ibm.com, linux-fsdevel@...r.kernel.org,
	sfrench@...ibm.com, philippe.deniel@....FR,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V7 4/9] vfs: Add open by file handle support

On Thu, 13 May 2010 16:09:55 +1000, Dave Chinner <david@...morbit.com> wrote:
> On Thu, May 13, 2010 at 09:44:22AM +1000, Neil Brown wrote:
> > On Wed, 12 May 2010 21:20:39 +0530
> > "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:
> > 
> > > +		filp->f_flags |= O_NOATIME;
> > > +		filp->f_mode |= FMODE_NOCMTIME;
> > > +	}
> > 
> > I think you need a comment here explaining the rational for these setting.
> 
> If you've never seen how applications use the XFS handle interface
> in conjunction with other XFS functionality, then I guess if would
> seem like bad voodoo.
> 
> > Why is O_NOATIME important IFREG but not for IFDIR?
> 
> No application has ever required directory access or modification
> via the handle interface to be invisible to the rest of the system.
> 
> > Why is it not sufficient to honour O_NOATIME that is passed in.
> 
> Because the XFS handle library is cross platform and predates
> O_NOATIME on linux. Hence the library it has never set that flag and
> always relied on the kernel implementation of the API to ensure
> atime was never updated on fds derived from handles..
> 
> > How can you ever justify setting FMODE_NOCMTIME ?
> 
> Quite easily. ;)
> 
> The XFS handle interface was designed specifically to allow
> applications to execute silent/invisible movement of data in, out
> and around the filesystem without leaving user visible traces in
> file metadata. This enables backup or filesysetm utilities that
> operate on active filesystems need to be able to access or modify
> inodes and data without affecting running applications.  It's a
> feature of the handle interface, and used by xfs_dump, xfs_fsr,
> SGI's HSM, etc to do stuff that isn't otherwise possible.
> 
> FWIW, if you are curious, here's the initial commit of the XFS
> handle code into Irix tree from 3 Sep 1994, showing that the initial
> XFS open_by_handle() implementation sets the FINVIS flag to trigger
> invisible IO semantics:
> 
> http://oss.sgi.com/cgi-bin/gitweb.cgi?p=archive/xfs-import.git;a=commitdiff;h=575b66fae833429a51fcadb204d45521c2dfc26f

Thanks for sharing this. I haven't looked at the details you mentioned here.

> 
> > I guess you are just copying from xfs code, but it still needs justification.
> 
> 	"They are intended for use by a limited set of system
> 	utilities such as backup programs."
> 
> 		- open_by_handle(3) man page
> 

Should we retain all the above behaviour in the new syscall ?. Or just
do what a normal open(2) call does ?

-aneesh
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