Re: [PATCH 4/9] xfs: ensure log tail is always up to date

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Aug 26, 2022 at 02:39:50PM -0700, Darrick J. Wong wrote:
> On Tue, Aug 23, 2022 at 12:18:47PM +1000, Dave Chinner wrote:
> > On Mon, Aug 22, 2022 at 05:33:19PM -0700, Darrick J. Wong wrote:
> > > On Wed, Aug 10, 2022 at 09:03:48AM +1000, Dave Chinner wrote:
> > > > From: Dave Chinner <dchinner@redhat.com>
> > > > 
> > > > Whenever we write an iclog, we call xlog_assign_tail_lsn() to update
> > > > the current tail before we write it into the iclog header. This
> > > > means we have to take the AIL lock on every iclog write just to
> > > > check if the tail of the log has moved.
> > > > 
> > > > This doesn't avoid races with log tail updates - the log tail could
> > > > move immediately after we assign the tail to the iclog header and
> > > > hence by the time the iclog reaches stable storage the tail LSN has
> > > > moved forward in memory. Hence the log tail LSN in the iclog header
> > > > is really just a point in time snapshot of the current state of the
> > > > AIL.
> > > > 
> > > > With this in mind, if we simply update the in memory log->l_tail_lsn
> > > > every time it changes in the AIL, there is no need to update the in
> > > > memory value when we are writing it into an iclog - it will already
> > > > be up-to-date in memory and checking the AIL again will not change
> > > > this.
> > > 
> > > This is too subtle for me to understand -- does the codebase
> > > already update l_tail_lsn?  Does this patch make it do that?
> > 
> > tl;dr: if the AIL is empty, log->l_tail_lsn is not updated on the
> > first insert of a new item into the AILi and hence is stale.
> > xlog_state_release_iclog() currently works around that by calling
> > xlog_assign_tail_lsn() to get the tail lsn from the AIL. This change
> > makes sure log->l_tail_lsn is always up to date.
> > 
> > In more detail:
> > 
> > The tail update occurs in xfs_ail_update_finish(), but only if we
> > pass in a non-zero tail_lsn. xfs_trans_ail_update_bulk() will only
> > set a non-zero tail_lsn if it moves the log item at the tail of the
> > log (i.e. we relog the tail item and move it forwards in the AIL).
> > 
> > Hence if we pass a non-zero tail_lsn to xfs_ail_update_finish(), it
> > indicates it needs to check it against the LSN of the item currently
> > at the tail of the AIL. If the tail LSN has not changed, we do
> > nothing, if it has changed, then we call
> > xlog_assign_tail_lsn_locked() to update the log tail.
> > 
> > The problem with the current code is that if the AIL is empty when
> > we insert the first item, we've actually moved the log tail but we
> > do not update the log tail (i.e. tail_lsn is zero in this case). If
> > we then release an iclog for writing at this point in time, the tail
> > lsn it writes into the iclog header would be wrong - it does not
> > reflect the log tail as defined by the AIL and the checkpoint that
> > has just been committed.
> > 
> > Hence xlog_state_release_iclog() called xlog_assign_tail_lsn() to
> > ensure that it checked that the tail LSN it applies to the iclog
> > reflects the current state of the AIL. i.e. it checks if there is an
> > item in the AIL, and if so, grabs the tail_lsn from the AIL. This
> > works around the fact the AIL doesn't update the log tail on the
> > first insert.
> > 
> > Hence what this patch does is have xfs_trans_ail_update_bulk set
> > the tail_lsn passed to xfs_ail_update_finish() to NULLCOMMITLSN when
> > it does the first insert into the AIL. NULLCOMMITLSN is a
> > non-zero value that won't match with the LSN of items we just
> > inserted into the AIL, and hence xfs_ail_update_finish() will go an
> > update the log tail in this case.
> > 
> > Hence we close the hole when the log->l_tail_lsn is incorrect after
> > the first insert into the AIL, and hence we no longer need to update
> > the log->l_tail_lsn when reading it into the iclog header -
> > log->l_tail_lsn is always up to date, and so we can now just read it
> > in xlog_state_release_iclog() rather than having to grab the AIL
> > lock and checking the AIL to update log->l_tail_lsn with the correct
> > tail value from iclog IO submission....
> 
> Ahhh, ok, I get it now.  Thanks for the explanation.

Looks ok to me now,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

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