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List:       linux-nfs
Subject:    Re: unlink within an open directory stream
From:       Jeff Layton <jlayton () redhat ! com>
Date:       2012-04-04 15:35:39
Message-ID: 20120404113539.2dc9b1dc () corrin ! poochiereds ! net
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On Mon, 26 Mar 2012 11:17:18 -0700
Boaz Harrosh <bharrosh@panasas.com> wrote:

> On 03/24/2012 10:12 AM, Myklebust, Trond wrote:
> 
> > On Sat, 2012-03-24 at 12:53 -0400, Matt W. Benjamin wrote:
> >> Hi,
> >>
> >> I don't think anything is.  Or, people originally reported the behavior against knfsd.
> >>
> >> Matt
> > 
> > There is a known issue with ext2/3/4 generating non-unique readdir
> > cookies. It rarely hits you when you are creating small directories, but
> > it frequently hits you with larger ones. A fix is underway that should
> > significantly reduce the frequency of cookie collisions.
> > 
> > Recent NFS clients will actually detect the presence of those cookie
> > loops, and log them in the kernel syslog. That would therefore be the
> > first thing that I'd check if confronted with this kind of problem.
> > 
> > Cheers
> >   Trond
> > 
> 
> 
> Trond please look on the bug report links below. It's not the "cookie collisions" case.
> 
> It's the new (post RHEL 6.0 Kernel) NFS need for opendir after an unlink.
> Now the POSIX man page *does* say that applications must re-opendir after
> unlink, but there are some applications who did not read the manual, and since
> it works with local filesystems and old nfs, (What Kernel RHEL 6.0 is based on?)
> they never noticed the bug and never fixed it.
> 

	^^^^^
Can you tell me which manpage says this? I'd like to be able to point
application developers at it if possible...

> Could we easily support the broken application by being bug compatible to
> old NFS versions?
> .i.e Don't require re-opendir after unlink of a file.
> 
> There are more examples in the bug reports below but basically bonnie++
> does the following:
> 	DIR *d = opendir(".");
> 	dirent *file_ent;
> 	while((file_ent = readdir(d)) != NULL) {
> 		unlink( file_ent->d_name))
> 	}
> 	closedir(d);
> 
> where it actually needs to do:
> 
> 	DIR *d = opendir(".");
> 	dirent *file_ent;
> 	while((file_ent = readdir(d)) != NULL) {
> 		unlink( file_ent->d_name))
> 
> 		closedir(d);
> 		d = opendir(".");
> 	}
> 	closedir(d);
> 
> But again case one used to work with old NFS. And it looks like
> it is not Server dependent. We saw this both with Ganesha as well
> as knfsd
> 
> <snip>
> 

Again, my suspicion is that the change that triggered this is the
switch to use READDIRPLUS on larger directories. Before that, we'd use
READDIR on larger ones and wouldn't need to make as many RPCs to fetch
directory contents. More continuation READDIRPLUS calls means that you
have more opportunity to hit problems with cookies.

What might be an interesting test is to see whether this is still
reproducible on newer clients when you mount with '-o nordirplus'.

Cheers,
-- 
Jeff Layton <jlayton@redhat.com>
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