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

[NeilBrown]

On Fri, 09 Sep 2022, Jeff Layton wrote:
> 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.

How do we know which inodes need the crash counter merged in?  I thought
the whole point of the crash counter was that it affected every file
(easy, safe, expensive, but hopefully rare enough that the expense could
be justified).

NeilBrown


> 
> > > 
> > > 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@xxxxxxxxxx>
> > > 
> 
> -- 
> Jeff Layton <jlayton@xxxxxxxxxx>
>