Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2022-09-09 at 09:01 +1000, NeilBrown wrote:
> On Fri, 09 Sep 2022, Jeff Layton wrote:
> > On Thu, 2022-09-08 at 14:22 -0400, J. Bruce Fields wrote:
> > > On Thu, Sep 08, 2022 at 01:40:11PM -0400, Jeff Layton wrote:
> > > > Yeah, ok. That does make some sense. So we would mix this into the
> > > > i_version instead of the ctime when it was available. Preferably, we'd
> > > > mix that in when we store the i_version rather than adding it afterward.
> > > > 
> > > > Ted, how would we access this? Maybe we could just add a new (generic)
> > > > super_block field for this that ext4 (and other filesystems) could
> > > > populate at mount time?
> > > 
> > > Couldn't the filesystem just return an ino_version that already includes
> > > it?
> > > 
> > 
> > Yes. That's simple if we want to just fold it in during getattr. If we
> > want to fold that into the values stored on disk, then I'm a little less
> > clear on how that will work.
> > 
> > Maybe I need a concrete example of how that will work:
> > 
> > Suppose we have an i_version value X with the previous crash counter
> > already factored in that makes it to disk. We hand out a newer version
> > X+1 to a client, but that value never makes it to disk.
> 
> As I understand it, the crash counter would NEVER appear in the on-disk
> i_version.
> The crash counter is stable while a filesystem is mounted so is the same
> when loading an inode from disk and when writing it back.
> 
> When loading, add crash counter to on-disk i_version to provide
> in-memory i_version.
> when storing, subtract crash counter from in-memory i_version to provide
> on-disk i_version.
> 
> "add" and "subtract" could be any reversible hash, and its inverse.  I
> would probably shift the crash counter up 16 and add/subtract.
> 
> 

If you store the value with the crash counter already factored-in, then
not every inode would end up being invalidated after a crash. If we try
to mix it in later, the client will end up invalidating the cache even
for inodes that had no changes.

> > 
> > The machine crashes and comes back up, and we get a query for i_version
> > and it comes back as X. Fine, it's an old version. Now there is a write.
> > What do we do to ensure that the new value doesn't collide with X+1? 
> > -- 
> > Jeff Layton <jlayton@kernel.org>
> > 

-- 
Jeff Layton <jlayton@kernel.org>