Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

[Jeff Layton]

On Thu, 2017-03-30 at 08:47 +0200, Jan Kara wrote:
> On Wed 29-03-17 13:54:31, Jeff Layton wrote:
> > On Wed, 2017-03-29 at 13:15 +0200, Jan Kara wrote:
> > > On Tue 21-03-17 14:46:53, Jeff Layton wrote:
> > > > On Tue, 2017-03-21 at 14:30 -0400, J. Bruce Fields wrote:
> > > > > On Tue, Mar 21, 2017 at 01:23:24PM -0400, Jeff Layton wrote:
> > > > > > On Tue, 2017-03-21 at 12:30 -0400, J. Bruce Fields wrote:
> > > > > > > - It's durable; the above comparison still works if there were reboots
> > > > > > >   between the two i_version checks.
> > > > > > > 	- I don't know how realistic this is--we may need to figure out
> > > > > > > 	  if there's a weaker guarantee that's still useful.  Do
> > > > > > > 	  filesystems actually make ctime/mtime/i_version changes
> > > > > > > 	  atomically with the changes that caused them?  What if a
> > > > > > > 	  change attribute is exposed to an NFS client but doesn't make
> > > > > > > 	  it to disk, and then that value is reused after reboot?
> > > > > > > 
> > > > > > 
> > > > > > Yeah, there could be atomicity there. If we bump i_version, we'll mark
> > > > > > the inode dirty and I think that will end up with the new i_version at
> > > > > > least being journalled before __mark_inode_dirty returns.
> > > > > 
> > > > > So you think the filesystem can provide the atomicity?  In more detail:
> > > > > 
> > > > 
> > > > Sorry, I hit send too quickly. That should have read:
> > > > 
> > > > "Yeah, there could be atomicity issues there."
> > > > 
> > > > I think providing that level of atomicity may be difficult, though
> > > > maybe there's some way to make the querying of i_version block until
> > > > the inode update has been journalled?
> > > 
> > > Just to complement what Dave said from ext4 side - similarly as with XFS
> > > ext4 doesn't guarantee atomicity unless fsync() has completed on the file.
> > > Until that you can see arbitrary combination of data & i_version after the
> > > crash. We do take care to keep data and metadata in sync only when there
> > > are security implications to that (like exposing uninitialized disk blocks)
> > > and if not, we are as lazy as we can to improve performance...
> > > 
> > > 
> > 
> > Yeah, I think what we'll have to do here is ensure that those
> > filesystems do an fsync prior to reporting the i_version getattr
> > codepath. It's not pretty, but I don't see a real alternative.
> 
> Hum, so are we fine if i_version just changes (increases) for all inodes
> after a server crash? If I understand its use right, it would mean
> invalidation of all client's caches but that is not such a big deal given
> how frequent server crashes should be, right?
> 
> Because if above is acceptable we could make reported i_version to be a sum
> of "superblock crash counter" and "inode i_version". We increment
> "superblock crash counter" whenever we detect unclean filesystem shutdown.
> That way after a crash we are guaranteed each inode will report new
> i_version (the sum would probably have to look like "superblock crash
> counter" * 65536 + "inode i_version" so that we avoid reusing possible
> i_version numbers we gave away but did not write to disk but still...).
> Thoughts?
> 

That does sound like a good idea. This is a 64 bit value, so we should
be able to carve out some upper bits for a crash counter without risking
wrapping.

The other constraint here is that we'd like any later version of the
counter to be larger than any earlier value that was handed out. I think
this idea would still satisfy that.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>