linux-kernel - Re: [RFC 0/3] Convert XFS inode hashes to radix trees

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Date:	Wed, 04 Oct 2006 15:22:19 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	David Chinner <dgc@....com>
Cc:	Chris Wedgwood <cw@...f.org>, xfs-dev@....com, xfs@....sgi.com,
	dhowells@...hat.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 0/3] Convert XFS inode hashes to radix trees

On Wed, 2006-10-04 at 08:22 +1000, David Chinner wrote:
> On Tue, Oct 03, 2006 at 02:23:35PM -0700, Chris Wedgwood wrote:
> > On Tue, Oct 03, 2006 at 04:06:10PM +1000, David Chinner wrote:
> > > Overall, the patchset removes more than 200 lines of code from the
> > > xfs inode caching and lookup code and provides more consistent
> > > scalability for large numbers of cached inodes. The only down side
> > > is that it limits us to 32 bit inode numbers of 32 bit platforms due
> > > to the way the radix tree uses unsigned longs for it's indexes
> > 
> >     commit afefdbb28a0a2af689926c30b94a14aea6036719
> >     tree 6ee500575cac928cd90045bcf5b691cf2b8daa09
> >     parent 1d32849b14bc8792e6f35ab27dd990d74b16126c
> >     author David Howells <dhowells@...hat.com> 1159863226 -0700
> >     committer Linus Torvalds <torvalds@...osdl.org> 1159887820 -0700
> > 
> >     [PATCH] VFS: Make filldir_t and struct kstat deal in 64-bit inode numbers
> > 
> >     These patches make the kernel pass 64-bit inode numbers internally when
> >     communicating to userspace, even on a 32-bit system.  They are required
> >     because some filesystems have intrinsic 64-bit inode numbers: NFS3+ and XFS
> >     for example.  The 64-bit inode numbers are then propagated to userspace
> >     automatically where the arch supports it.
> >     [...]
> > 
> > Doing this will mean XFS won't be able to support 32-bit inodes on
> > 32-bit platforms the above (merged) patch --- though given that cheap
> > 64-bit systems are now abundant does anyone really care?

Which completely ignored the fact that NFS systems are already having to
truncate 64-bit inode numbers to 32-bits and pass these truncated values
up to userspace. Collisions have been observed in the wild, and I've
already had to change the 64-bit->32-bit hashing algorithm on at least
one occasion.

By moving that truncation into userspace, we will at least give 64-bit
standards-compliant programs a chance to work correctly.

Trond

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