On Tue, 2022-08-30 at 10:08 +1000, Dave Chinner wrote: > On Mon, Aug 29, 2022 at 06:33:48AM -0400, Jeff Layton wrote: > > On Mon, 2022-08-29 at 15:48 +1000, Dave Chinner wrote: > > > > > > > > The race window ought to be relatively small, and this wouldn't result > > > > in incorrect behavior that you'd notice (other than loss of > > > > performance), but it's not ideal. We're doing more on-the-wire reads > > > > than are necessary in this case. > > > > > > > > It would be nice to have it not do that. If we end up taking this patch > > > > to make it elide the i_version bumps on atime updates, we may be able to > > > > set the the NOIVER flag in other cases as well, and avoid some of these > > > > extra bumps. > > > > > > > > > <sigh> > > > > > > Please don't make me repeat myself for the third time. > > > > > > Once we have decided on a solid, unchanging definition for the > > > *statx user API variable*, we'll implement a new on-disk field that > > > provides this information. We will document it in the on-disk > > > specification as "this is how di_iversion behaves" so that it is > > > clear to everyone parsing the on-disk format or writing their own > > > XFS driver how to implement it and when to expect it to > > > change. > > > > > > Then we can add a filesystem and inode feature flags that say "inode > > > has new iversion" and we use that to populate the kernel iversion > > > instead of di_changecount. We keep di_changecount exactly the way it > > > is now for the applications and use cases we already have for that > > > specific behaviour. If the kernel and/or filesystem don't support > > > the new di_iversion field, then we'll use di_changecount as it > > > currently exists for the kernel iversion code. > > > > > > > Aside from NFS and IMA, what applications are dependent on the current > > definition and how do they rely on i_version today? > > I've answered this multiple times already: the di_changecount > behaviour is defined in the on-disk specification and hence we > *cannot change the behaviour* without changing the on-disk format > specification. > > Apart from the forensics aspect of the change counter (which nobody > but us XFS developers seem to understand just how damn important > this is), there are *many* third party applications that parse the > XFS on-disk format directly. This: > > https://codesearch.debian.net/search?q=XFS_SB_VERSION_DIRV2&literal=1 > > Shows grub2, libparted, syslinux, partclone and fsarchiver as > knowing about XFS on-disk superblock flags that tell them what > format the directory structure is in. That alone is enough to > indicate they parse on-disk inodes directly, and hence may expect > di_changecount to have specific meaning and use it to detect > unexpected changes to files/directories they care about. > > If I go looking for XFS_SB_MAGIC, I find things like libblkid, > klibc, qemu, Xen, testdisk, gpart, and virtualbox all parse the > on-disk superblocks directly from the block device, too. They also > rely directly on XFS developers ensuring there are no silent > incomaptible changes to the on disk format. > > I also know of many other utilities that people and companies have > written that parse the on disk format directly from userspace. The > functions these perform include low level storage management tools, > copying and managing disk images (e.g. offline configuration for > cluster deployments), data recovery tools that scrape all the data > out of broken filesystems, etc. > > These applications are reliant on the guarantee we provide that the > on-disk format will not silently change and that behaviour/structure > can always easily be discovered by feature flags in the superblock > and/or inodes. > > IOWs, just because there aren't obvious "traditional" application on > top of the kernel filesystem that consumes the in-memory kernel > iversion field, it does not mean that the defined behaviour of the > on-disk di_changecount field is not used or relied on by other tools > that work directly on the on-disk format. > > You might be right that NFS doesn't care about this, but the point > remains that NFS does not control the XFS on-disk format, nor does > the fact that what NFS wants from the change attribute has changed > over time override the fact that maintaining XFS on-disk format > compatibility is the responsibility of XFS developers. We're willing > to change the on-disk format to support whatever the new definition > of the statx change attribute ends up being, and that should be the > end of the discussion. > Thanks for spelling this out in more detail. > > > Keep in mind that we've been doing dynamic inode format updates in > > > XFS for a couple of decades - users don't even have to be aware that > > > they need to perform format upgrades because often they just happen > > > whenever an inode is accessed. IOWs, just because we have to change > > > the on-disk format to support this new iversion definition, it > > > doesn't mean users have to reformat filesystems before the new > > > feature can be used. > > > > > > Hence, over time, as distros update kernels, the XFS iversion > > > behaviour will change automagically as we update inodes in existing > > > filesystems as they are accessed to add and then use the new > > > di_iversion field for the VFS change attribute field instead of the > > > di_changecount field... > > > > > > > If you want to create a whole new on-disk field for this, then that's > > your prerogative, but before you do that, I'd like to better understand > > why and how the constraints on this field changed. > > > > The original log message from the commit that added a change counter > > (below) stated that you were adding it for network filesystems like NFS. > > When did this change and why? > > It never changed. I'll repeat what I've already explained twice > before: > > https://lore.kernel.org/linux-xfs/20220818030048.GE3600936@xxxxxxxxxxxxxxxxxxx/ > https://lore.kernel.org/linux-xfs/20220818033731.GF3600936@xxxxxxxxxxxxxxxxxxx/ > > tl; dr: NFS requirements were just one of *many* we had at the time > for an atomic persistent change counter. > > The fact is that NFS users are just going to have to put up with > random cache invalidations on XFS for a while longer. Nobody noticed > this and/or cared about this enough to raise it as an issue for the > past decade, so waiting another few months for upstream XFS to > change to a different on-disk format for the NFS/statx change > attribute isn't a big deal. > Fair enough. I'll plan to drop this patch from the series for now, with the expectation that you guys will add a new i_version counter that better conforms to what NFS and IMA need. Thanks, -- Jeff Layton <jlayton@xxxxxxxxxx>
