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

[Dave Chinner]

On Wed, Mar 29, 2017 at 01:54:31PM -0400, 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.

I think that's even more problematic. ->getattr currently runs
completely unlocked for performance reasons - it's racy w.r.t. to
ongoing modifications to begin with, so /nothing/ that is returned
to userspace via stat/statx can be guaranteed to be "coherent".
Linus will be very unhappy if you make his git workload (which is
/very/ stat heavy) run slower by adding any sort of locking in this
hot path.

Even if we did put an fsync() into ->getattr() (and dealt with all
the locking issues that entails), by the time the statx syscall
returns to userspace the i_version value may not match the
data/metadata in the inode(*).  IOWs, by the time i_version gets
to userspace, it is out of date and any use of it for data
versioning from userspace is going to be prone to race conditions.

Cheers,

Dave.

(*) fiemap has exactly the same "stale the moment internal fs
locks are released" race conditions, which is why it cannot safely
be used for mapping holes when copying file data....

-- 
Dave Chinner
david@xxxxxxxxxxxxx