On Thu 08-09-22 10:44:22, NeilBrown wrote: > On Wed, 07 Sep 2022, Jan Kara wrote: > > On Wed 07-09-22 09:12:34, Jeff Layton wrote: > > > On Wed, 2022-09-07 at 08:52 -0400, J. Bruce Fields wrote: > > > > On Wed, Sep 07, 2022 at 08:47:20AM -0400, Jeff Layton wrote: > > > > > On Wed, 2022-09-07 at 21:37 +1000, NeilBrown wrote: > > > > > > On Wed, 07 Sep 2022, Jeff Layton wrote: > > > > > > > +The change to \fIstatx.stx_ino_version\fP is not atomic with respect to the > > > > > > > +other changes in the inode. On a write, for instance, the i_version it usually > > > > > > > +incremented before the data is copied into the pagecache. Therefore it is > > > > > > > +possible to see a new i_version value while a read still shows the old data. > > > > > > > > > > > > Doesn't that make the value useless? > > > > > > > > > > > > > > > > No, I don't think so. It's only really useful for comparing to an older > > > > > sample anyway. If you do "statx; read; statx" and the value hasn't > > > > > changed, then you know that things are stable. > > > > > > > > I don't see how that helps. It's still possible to get: > > > > > > > > reader writer > > > > ------ ------ > > > > i_version++ > > > > statx > > > > read > > > > statx > > > > update page cache > > > > > > > > right? > > > > > > > > > > Yeah, I suppose so -- the statx wouldn't necessitate any locking. In > > > that case, maybe this is useless then other than for testing purposes > > > and userland NFS servers. > > > > > > Would it be better to not consume a statx field with this if so? What > > > could we use as an alternate interface? ioctl? Some sort of global > > > virtual xattr? It does need to be something per-inode. > > > > I was thinking how hard would it be to increment i_version after updating > > data but it will be rather hairy. In particular because of stuff like > > IOCB_NOWAIT support which needs to bail if i_version update is needed. So > > yeah, I don't think there's an easy way how to provide useful i_version for > > general purpose use. > > > > Why cannot IOCB_NOWAIT update i_version? Do we not want to wait on the > cmp_xchg loop in inode_maybe_inc_iversion(), or do we not want to > trigger an inode update? > > The first seems unlikely, but the second seems unreasonable. We already > acknowledge that after a crash iversion might go backwards and/or miss > changes. It boils down to the fact that we don't want to call mark_inode_dirty() from IOCB_NOWAIT path because for lots of filesystems that means journal operation and there are high chances that may block. Presumably we could treat inode dirtying after i_version change similarly to how we handle timestamp updates with lazytime mount option (i.e., not dirty the inode immediately but only with a delay) but then the time window for i_version inconsistencies due to a crash would be much larger. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
