On Mon, Aug 14, 2017 at 11:11:41AM +0300, Alex Lyakas wrote: > Hello David, Brian, > > I was not able to follow the details, unfortunately. Can you confirm that > this patch is safe to go into kernel 3.18? > This is the open question in the separate subthread (this one is discussion around designing a solution for the current code): http://marc.info/?l=linux-xfs&m=150246184413604&w=2 This could use confirmation, but my understanding is that this is safe because v3.18 doesn't have the more advanced deferred ops infrastructure. It uses xfs_bmap_finish() which has a max roll count of one and a transaction with enough reservation for 2 rolls before blocking reservation is required. Note that doesn't mean we'd officially post a v3.18 stable patch before this is fixed in the upstream code. We always fix upstream first and backport from there to ensure a consistent base going forward (we don't want to go change v3.18, end up with a slightly different upstream patch, then have to backport more changes to fix the original patch). This may be safe enough for you to use locally in the meantime, however. Brian > Thanks, > Alex. > > > -----Original Message----- From: Dave Chinner > Sent: Monday, August 14, 2017 3:28 AM > To: Brian Foster > Cc: Darrick J. Wong ; Alex Lyakas ; linux-xfs@xxxxxxxxxxxxxxx ; > libor.klepac@xxxxxxx > Subject: Re: [PATCH] xfs: hold xfs_buf locked between shortform->leaf > conversion and the addition of an attribute > > On Sat, Aug 12, 2017 at 10:04:34AM -0400, Brian Foster wrote: > > On Sat, Aug 12, 2017 at 10:16:37AM +1000, Dave Chinner wrote: > > > On Fri, Aug 11, 2017 at 10:27:43AM -0400, Brian Foster wrote: > > > > On Fri, Aug 11, 2017 at 12:22:04PM +1000, Dave Chinner wrote: > > > Using XFS_BLI_ORDERED allows us to log the buffer without recording > > > a new dirty range on the buffer. IOWs, it retains whatever dirty range > > > it already had, and so after joining, marking it ordered and then > > > logging the buffer, we have a XFS_BLI_DIRTY | XFS_BLI_ORDERED buffer > > > in the transaction. > > > > > > The question is this: what happens when a XFS_BLI_ORDERED buffer > > > with a pre-existing dirty region is formatted for the CIL? We > > > haven't done that before, so I'm betting that we don't relog the > > > dirty region like we should be doing.... > > > > > > ... and we don't relog the existing dirty range because the > > > ordered flag takes precedence. > > > > > > > Right.. so it seems that the current implementation for ordered buffers > > assumes a buffer is only ever used in one mode or the other. > > Additionally, the AIL assumes that any reinserted item has been fully > > relogged and so it moves the LSN forward unconditionally. Current > > ordered buffer processing violates this constraint for an already logged > > buffer. > > Right, but it's not been a concern until now because we've only ever > used ordered buffers on newly allocated buffers that haven't been > previously logged. > > > > Ok, the ordered buffer checks in xfs_buf_item_size() and > > > xfs_buf_item_format() need to also check for dirty regions. If dirty > > > regions exist, then we treat it like a normal buffer rather than an > > > ordered buffer. We can factor the dirty region check out of > > > xfs_buf_item_unlock() for this... > > > > > > Actually, check the case in xfs_buf_item_size() and remove the > > > ordered flag if there are dirty regions. Then xfs_buf_item_format() > > > will do the right thing without needing a duplicate check... > > > > > > > I think that would work, assuming we actually check the > > xfs_buf_log_format for dirty-ness rather than just the log item. As it > > is, note that ordered buffers are still "logged" in the transaction > > because otherwise the transaction infrastructure will assume it made no > > change to the buf and toss the log item at commit time (we also need to > > set up I/O completion on the buf and whatnot). > > *nod* > > > What concerns me about this approach is that I think we introduce the > > possibility for subtle bugs. Existing ordered buffer code does this: > > > > xfs_trans_ordered_buf(tp, fbuf); > > xfs_trans_log_buf(tp, fbuf, 0, > > BBTOB(fbuf->b_length) - 1); > > > > ... which should continue to work fine. Allowing ordered buffers to > > physically log means that something like this: > > > > xfs_trans_log_buf(tp, fbuf, 0, > > BBTOB(fbuf->b_length) - 1); > > xfs_trans_ordered_buf(tp, fbuf); > > > > ... is now a bug that is only apparent after scrutiny of xfs_trans_*() > > and logging internals. Granted, the above already is incorrect, but it > > technically still works as expected. I don't see the need to turn that > > into a real problem by actually logging the buffer when we might not > > expect to. > > Well, it's not a "things go bad" bug. It's a "we screwed up an > optimisation" bug, because logging the buffer contents unnecessarily > only increases the required log bandwidth. It shouldn't affect > replay because the buffer is still correctly ordered in the log. > Hence both the transient and end states of the buffer during replay > will still be the same... > > > So while I agree that this could probably be made to work and I think it > > is ideal to doing any kind of logged range tracking in the deferred ops > > code, it still seems more tricky than it needs to be. To relog a held > > buffer in a new transaction, why not just mark the lidp dirty in the new > > transaction so it inherits all existing dirty segments? AFAICT, all we > > really need to do is: > > > > tp->t_flags |= XFS_TRANS_DIRTY; > > lidp->lid_flags |= XFS_LID_DIRTY; > > > > ... on the new transaction and everything should just work as designed > > (for a buffer that has been previously logged, held, rolled and > > rejoined). > > We would also need to set: > > bip->bli_flags |= XFS_BLI_DIRTY | XFS_BLI_LOGGED; > > which means we should.... > > > To elaborate a bit, I think we could refactor xfs_trans_log_buf() into a > > new xfs_trans_dirty_buf() helper that covers all of the relevant bits > > not related to actually dirtying the bli. xfs_trans_log_buf() would call > > xfs_trans_dirty_buf() and thus would not change functionally. > > xfs_trans_ordered_buf() could now call xfs_trans_dirty_buf() and thus > > the existing ordered buf users would no longer need to log a range of > > the buffer (which doesn't make much sense anyways). > > ... do this. :) > > > Finally, the > > deferred infrastructure could join/dirty/hold the buffer to the new > > transaction after each roll without needing to track and relog specific > > regions of the buffer. Thoughts? > > Yup, that's exactly what I was thinking should be possible by using > ordered buffers.... :) > > And Christoph's rework of the transaction roll and deferred inode > handling that he just posted should make adding buffer handling > quite a bit neater and cleaner. > > > Unless I'm missing something as to why this is busted, I'll take a > > closer look at the code and float an rfc next week since otherwise it > > sounds like this is something we could actually fix up in the ordered > > buffer code today. > > Cool. > > > > Nothing in XFS is ever simple, is it? :P > > > > There used to be a level of satisfaction at feeling I understood some > > new corner of XFS. Nowadays I know that just means I'm not yet aware of > > whatever dragons remain in that corner (is that paranoia? not if it's > > true!). :P > > Ah, the true signs of expertise: developing a knowledge base and > insight deep enough to understand that there is always another > hidden dragon poised to bite your head off. :) > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx > > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
