On Wed, Jun 03, 2020 at 07:29:18AM +1000, Dave Chinner wrote: > On Tue, Jun 02, 2020 at 12:45:35PM -0400, Brian Foster wrote: > > On Tue, Jun 02, 2020 at 07:42:25AM +1000, Dave Chinner wrote: > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > > > Inode buffers always have write IO callbacks, so by marking them > > > directly we can avoid needing to attach ->b_iodone functions to > > > them. This avoids an indirect call, and makes future modifications > > > much simpler. > > > > > > This is largely a rearrangement of the code at this point - no IO > > > completion functionality changes at this point, just how the > > > code is run is modified. > > > > > > > Ok, I was initially thinking this patch looked incomplete in that we > > continue to set ->b_iodone() on inode buffers even though we'd never > > call it. Looking ahead, I see that the next few patches continue to > > clean that up to eventually remove ->b_iodone(), so that addresses that. > > > > My only other curiosity is that while there may not be any functional > > difference, this technically changes callback behavior in that we set > > the new flag in some contexts that don't currently attach anything to > > the buffer, right? E.g., xfs_trans_inode_alloc_buf() sets the flag on > > inode chunk init, which means we can write out an inode buffer without > > any attached/flushed inodes. > > Yes, it can happen, and it happens before this patch, too, because > the AIL can push the buffer log item directly and that does not > flush dirty inodes to the buffer before it writes back(*). > I was thinking more about cases where there are actually no inodes attached. > As it is, xfs_buf_inode_iodone() on a buffer with no inode attached > if functionally identical to the existing xfs_buf_iodone() callback > that would otherwise be done. i.e. it just runs the buffer log item > completion callback. Hence the change here rearranges code, but it > does not change behaviour at all. > Right. That's indicative from the code, but doesn't help me understand why the change is made. That's all I'm asking for... > (*) this is a double-write bug that this patch set does not address. > i.e. buffer log item flushes the buffer without flushing inodes, IO > compeletes, then inode flush to the buffer and we do another IO to > clean them. This is addressed by a follow-on patchset that tracks > dirty inodes via ordered cluster buffers, such that pushing the > buffer always triggers xfs_iflush_cluster() on buffers tagged > _XBF_INODES... > Ok, interesting (but seems beyond the scope of this series). > > Is the intent of that to support future > > changes? If so, a note about that in the commit log would be helpful. > > That's part of it, as you can see from the (*) above. But the commit > log already says "..., and makes future modifications much simpler." > Was that insufficient to indicate that it will be used later on? > That's a rather vague hint. ;P I was more hoping for something like: "While this is largely a refactor of existing functionality, broaden the scope of the flag to beyond where inodes are explicitly attached because <some actual reason>. This has the effect of possibly invoking the callback in cases where it wouldn't have been previously, but this is not a functional change because the callback is effectively a no-op when inodes are not attached." Brian > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx >
