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Date: Fri, 09 Sep 2022 09:01:16 +1000 From: "NeilBrown" <neilb@...e.de> To: "Jeff Layton" <jlayton@...nel.org> Cc: "J. Bruce Fields" <bfields@...ldses.org>, "Theodore Ts'o" <tytso@....edu>, "Jan Kara" <jack@...e.cz>, adilger.kernel@...ger.ca, djwong@...nel.org, david@...morbit.com, trondmy@...merspace.com, viro@...iv.linux.org.uk, zohar@...ux.ibm.com, xiubli@...hat.com, chuck.lever@...cle.com, lczerner@...hat.com, brauner@...nel.org, fweimer@...hat.com, linux-man@...r.kernel.org, linux-api@...r.kernel.org, linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field 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. 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@...nel.org> >
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