Re: [PATCH] xfs: hold xfs_buf locked between shortform->leaf conversion and the addition of an attribute

[Brian Foster <bfoster@redhat.com>]

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@vger.kernel.org ;
> libor.klepac@bcom.cz
> 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@fromorbit.com
> 
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