linux-ext4 - Re: [Gluster-devel] regressions due to 64-bit ext4 directory cookies

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Date:	Wed, 13 Feb 2013 18:05:11 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Anand Avati <anand.avati@...il.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>,
	sandeen@...hat.com, linux-nfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, gluster-devel@...gnu.org
Subject: Re: [Gluster-devel] regressions due to 64-bit ext4 directory cookies

On Wed, Feb 13, 2013 at 02:57:13PM -0800, Anand Avati wrote:
> On Wed, Feb 13, 2013 at 2:47 PM, Theodore Ts'o <tytso@....edu> wrote:
> 
> > On Wed, Feb 13, 2013 at 05:41:41PM -0500, J. Bruce Fields wrote:
> > > > What if we have an ioctl or a process personality flag where a broken
> > > > application can tell the file system "I'm broken, please give me a
> > > > degraded telldir/seekdir cookie"?  That way we don't penalize programs
> > > > that are doing the right thing, while providing some accomodation for
> > > > programs who are abusing the telldir cookie.
> > >
> > > Yeah, if there's a simple way to do that, maybe it would be worth it.
> >
> > Doing this as an ioctl which gets called right after opendir, i.e
> > (ignoring error checking):
> >
> >       DIR *dir = opendir("/foo/bar/baz");
> >       ioctl(dirfd(dir), EXT4_IOC_DEGRADED_READDIR, 1);
> >       ...
> >
> > should be quite easy.  It would be a very ext3/4 specific thing,
> > though.
> 
> 
> That would work, even though it would be ext3/4 specific. What is the
> recommended programmatic way to detect if the file is on ext3/4 -- we would
> not want to attempt that blindly on a non-ext3/4 FS as the numerical value
> of EXT4_IOC_DEGRADED_READDIR might get interpreted in dangerous ways?

We must have been through this before, but: is the only way to generate
a collision-free readdir cookie really to use a larger hash?

Would it be possible to make something work like, for example, a 31-bit
hash plus an offset into a hash bucket?

I have trouble thinking about this, partly because I can't remember
where to find the requirements for readdir on concurrently modified
directories....

--b.
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