linux-kernel - Re: [PATCH v2 1/5] fat: allocate persistent inode numbers

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Date:	Wed, 12 Sep 2012 13:11:28 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Namjae Jeon <linkinjeon@...il.com>,
	"Steven J. Magnani" <steve@...idescorp.com>,
	Al Viro <viro@...iv.linux.org.uk>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Namjae Jeon <namjae.jeon@...sung.com>,
	Ravishankar N <ravi.n1@...sung.com>,
	Amit Sahrawat <a.sahrawat@...sung.com>
Subject: Re: [PATCH v2 1/5] fat: allocate persistent inode numbers

On Thu, Sep 13, 2012 at 02:03:51AM +0900, OGAWA Hirofumi wrote:
> "J. Bruce Fields" <bfields@...ldses.org> writes:
> 
> >> > Supposing, the server/client state is after cold boot, and client try to
> >> > rename at first without any cache on client/server.
> >> >
> >> > Even if this state, does the server return ESTALE? If it doesn't return
> >> > ESTALE, I can't understand why it is really unfixable.
> >> Hi OGAWA.
> >> Server will not return ESTALE in this case. because the client does
> >> not have any information for files yet.
> >
> > It does if the client mounted before the server rebooted.  NFS is
> > designed so that servers can reboot without causing clients to fail.
> > (Applications will just see a delay during the reboot.)
> >
> > It probably isn't possible to this work in the case of fat.
> >
> > But from fat's point of view there probably isn't much difference
> > between a filehandle lookup after a reboot and a filehandle lookup after
> > the inode's gone from cache.
> 
> This is talking about to retry by client side, not server side solution.
> What happens if client retry after got ESTALE? (Yes, this would not be
> the solution for all NFS clients. But, I guess this solution can be for
> linux NFS client.)
> 
> > I really don't see what you can do to help here.  Won't anything that
> > allows looking up an uncached inode by filehandle also risk finding the
> > wrong file?
> 
> On other view (as server side solution), we are thinking there is
> possible to make the stable filehandle on FAT if we disabled some
> operations (e.g. rename(), unlink()) which change inode location in FAT.
> 
> Yes, it would be stable, but supporting limited operations.
> 
> This is server side solution, and we comparing it with client solution.

Is that useful to anyone?

> >> > If it returns ESTALE, why does it return? I'm assuming the previous code
> >> > path is the cached FH path.
> >> The main point for observation is the file handle-which is used for
> >> all the NFS operation.
> >> So for all the NFS operation(read/write....) which makes use of the
> >> NFS file handle in between if there is a change in inode number
> >> It will result in ESTALE.
> >> Changing inode number on rename happened at NFS server by inode cache
> >> eviction with memory pressure.
> >> 
> >> lookupcache is used at NFS client to reduce number of LOOKUP operations.
> >> But , we can still get ESTALE if inode number at NFS Server change
> >> after LOOKUP, although lookupcache is disable.
> >> 
> >> LOOKUP return NFS FH->[inode number changed at NFS Server] ->
> >> But we still use old NFS FH returned from LOOKUP for any file
> >> operation(write,read,etc..)
> >> -> ESTALE will be returned.
> 
> Yes.  And I'm expecting as client side solution,
> 
> -> ESTALE will be returned -> discard old FH -> restart from LOOKUP ->
> make cached inode -> use returned new FH.
> 
> Yeah, I know this is unstable (there is no perfect solution for now),

You may end up with a totally different file, of course:

	client:			server:

	open "/foo/bar"
				rename "/foo/baz"->"/foo/bar"
	write to file

And now we're writing to the file that was originally named /foo/baz
when we should have gotten ESTALE.

--b.

> but if this works, this doesn't limit any operations.
> 
> We would want to compare client solution (-mm) and server solution
> (stable ino). Or I'd like to know which my knowledges/understanding are
> wrong here.
> -- 
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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