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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Aug 10, 2017 at 10:52:49AM -0400, Brian Foster wrote:
> On Thu, Aug 10, 2017 at 03:09:09PM +0300, Alex Lyakas wrote:
> > Hi Dave,
> > 
> > Thanks for the explanation. So it seems we cannot move forward with this
> > fix.
> > 
> 
> I don't think this completely invalidates the fix.. Dave is pointing out
> a flaw that the deferred ops infrastructure doesn't properly handle the
> technique we want to use here. IOW, it means there's a dependency that
> needs to be implemented first.
> 
> FWIW, I also think this means that your approach on the older kernel to
> join/hold the buffer to the finished transaction may be the right
> approach (depending on whether I follow the perm transaction code
> correctly or not, see below), but I think you'd need to relog the buffer
> as well.
> 
> > Will somebody else in XFS community be working on fixing this issue? As you
> > pointed out, it exists for over two decades. Our production systems hit this
> > every couple of days, and shutting down the filesystem causes outage.
> > 
> 
> I'm guessing the defer infrastructure needs to handle relogging a buffer
> similar to how it currently handles joining/relogging inodes..?
> 
> ...
> > -----Original Message----- From: Dave Chinner
> ...
> > The problem is that the locked buffer is not joined and logged in
> > the rolling transactions run in xfs_defer_ops. Hence it can pin the
> > tail of the AIL, and this can prevent the transaction roll from
> > regranting the log space necessary to continue rolling the
> > transaction for the required number of transactions to complete the
> > deferred ops. If this happens, we end up with a log space deadlock.
> > 
> > Hence if we are holding an item that we logged in a transaction
> > locked and we roll the transaction, we have to join, hold and log it
> > in each subsequent transaction until we have finished with the item
> > and can unlock and release it.
> > 
> > This is documented in xfs_trans_roll():
> > 
> >        /*
> >         * Reserve space in the log for th next transaction.
> >         * This also pushes items in the "AIL", the list of logged items,
> >         * out to disk if they are taking up space at the tail of the log
> >         * that we want to use.  This requires that either nothing be locked
> >         * across this call, or that anything that is locked be logged in
> >         * the prior and the next transactions.
> >         */
> > 
> 
> Good catch, though I'm wondering whether it's a real enough problem atm
> to block this fix. A few thoughts/questions:
> 
> 1.) The transaction in this case has a t_log_count of 3, presumably to
> cover the commits by the historical bmap_finish, the trans roll and the
> final commit. If I'm following the permanent transaction code correctly,
> doesn't that mean that we have room for at least 2 rolls (and 3 commits)
> before this task would actually block on log reservation? AFAICT it
> looks like the commit would dec ticket->t_cnt and replenish the current
> log reservation. The subsequent xfs_trans_reserve() would just return if
> t_cnt > 0.
> 
> This of course doesn't accommodate the fact the xfs_defer_finish() can
> now roll a transaction an indeterminate number of times, which probably
> needs to be handled in general, but...

I'd been wondering if tr_logcount needed upward adjusting, but so far
haven't observed any problems.

> 2.) It doesn't look like we actually defer any ops in this situation
> unless rmapbt is enabled, assuming that we limit holding the buffer to
> the empty leaf case, at least (see my comment on the previous version).
> I also don't see where a deferred rmapbt update would itself ever roll
> the transaction.

rmapbt split causes the agfl to hit the low water mark and refresh,
requiring an allocation ... but I think that's all stuffed in the same
transaction.  (So yeah I think I agree with you)

> 3.) The buffer in this case is a new allocation, which I think means the
> risk of it pinning the tail and causing a log deadlock here means that
> on top of somehow depleting the initial permanent reservation, we'd have
> to exhaust the log in the time between the first commit and the last
> reservation.
> 
> Given the above, it seems reasonably safe enough to me to merge this
> change as is and fix up the deferred ops stuff after the fact
> (considering we know we need to rework the xattr stuff as such anyways).
> This is still a landmine that should be fixed up, but I wouldn't be
> against an ASSERT() or something for the time being if we could somehow
> verify that the transaction ticket didn't require any extra reservation.
> 
> OTOH, just adding deferred ops buffer relogging might not be too much
> trouble either. ;) Anyways, thoughts?

I don't think it'd be difficult to add a _defer_bjoin operation that
maintains a list of buffers that we need to bhold across rolls.

I think xfs_buf->b_list is only used for delwri buffers, and a buffer
cannot be part of a transaction /and/ on a delwri list at the same time,
right?  So it shouldn't be hard to whip something up and couple this
patch to that.

--D

> 
> Brian
> 
> > 
> > Cheers,
> > 
> > Dave.
> > 
> > -- 
> > Dave Chinner
> > david@fromorbit.com
> > 
> > --
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