Re: [PATCH v22 02/16] xfs: clear log incompat feature bits when the log is idle

[Allison Henderson <allison.henderson@xxxxxxxxxx>]

On 7/27/21 5:46 AM, Chandan Babu R wrote:
> On 27 Jul 2021 at 11:50, Allison Henderson wrote:
> > From: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> >
> > When there are no ongoing transactions and the log contents have been
> > checkpointed back into the filesystem, the log performs 'covering',
> > which is to say that it log a dummy transaction to record the fact that
> > the tail has caught up with the head.  This is a good time to clear log
> > incompat feature flags, because they are flags that are temporarily set
> > to limit the range of kernels that can replay a dirty log.
> >
> > Since it's possible that some other higher level thread is about to
> > start logging items protected by a log incompat flag, we create a rwsem
> > so that upper level threads can coordinate this with the log.  It would
> > probably be more performant to use a percpu rwsem, but the ability to
> > /try/ taking the write lock during covering is critical, and percpu
> > rwsems do not provide that.
>
> Looks good to me.
>
> Reviewed-by: Chandan Babu R <chandanrlinux@xxxxxxxxx>
Thank you!

Allison

> > Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> > Reviewed-by: Allison Henderson <allison.henderson@xxxxxxxxxx>
> > Signed-off-by: Allison Henderson <allison.henderson@xxxxxxxxxx>
> > ---
> >   fs/xfs/xfs_log.c      | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
> >   fs/xfs/xfs_log.h      |  3 +++
> >   fs/xfs/xfs_log_priv.h |  3 +++
> >   3 files changed, 55 insertions(+)
> >
> > diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> > index 9254405..c58a0d7 100644
> > --- a/fs/xfs/xfs_log.c
> > +++ b/fs/xfs/xfs_log.c
> > @@ -1338,6 +1338,32 @@ xfs_log_work_queue(
> >   }
> >   
> >   /*
> > + * Clear the log incompat flags if we have the opportunity.
> > + *
> > + * This only happens if we're about to log the second dummy transaction as part
> > + * of covering the log and we can get the log incompat feature usage lock.
> > + */
> > +static inline void
> > +xlog_clear_incompat(
> > +	struct xlog		*log)
> > +{
> > +	struct xfs_mount	*mp = log->l_mp;
> > +
> > +	if (!xfs_sb_has_incompat_log_feature(&mp->m_sb,
> > +				XFS_SB_FEAT_INCOMPAT_LOG_ALL))
> > +		return;
> > +
> > +	if (log->l_covered_state != XLOG_STATE_COVER_DONE2)
> > +		return;
> > +
> > +	if (!down_write_trylock(&log->l_incompat_users))
> > +		return;
> > +
> > +	xfs_clear_incompat_log_features(mp);
> > +	up_write(&log->l_incompat_users);
> > +}
> > +
> > +/*
> >    * Every sync period we need to unpin all items in the AIL and push them to
> >    * disk. If there is nothing dirty, then we might need to cover the log to
> >    * indicate that the filesystem is idle.
> > @@ -1363,6 +1389,7 @@ xfs_log_worker(
> >   		 * synchronously log the superblock instead to ensure the
> >   		 * superblock is immediately unpinned and can be written back.
> >   		 */
> > +		xlog_clear_incompat(log);
> >   		xfs_sync_sb(mp, true);
> >   	} else
> >   		xfs_log_force(mp, 0);
> > @@ -1450,6 +1477,8 @@ xlog_alloc_log(
> >   	}
> >   	log->l_sectBBsize = 1 << log2_size;
> >   
> > +	init_rwsem(&log->l_incompat_users);
> > +
> >   	xlog_get_iclog_buffer_size(mp, log);
> >   
> >   	spin_lock_init(&log->l_icloglock);
> > @@ -3895,3 +3924,23 @@ xfs_log_in_recovery(
> >   
> >   	return log->l_flags & XLOG_ACTIVE_RECOVERY;
> >   }
> > +
> > +/*
> > + * Notify the log that we're about to start using a feature that is protected
> > + * by a log incompat feature flag.  This will prevent log covering from
> > + * clearing those flags.
> > + */
> > +void
> > +xlog_use_incompat_feat(
> > +	struct xlog		*log)
> > +{
> > +	down_read(&log->l_incompat_users);
> > +}
> > +
> > +/* Notify the log that we've finished using log incompat features. */
> > +void
> > +xlog_drop_incompat_feat(
> > +	struct xlog		*log)
> > +{
> > +	up_read(&log->l_incompat_users);
> > +}
> > diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h
> > index 813b972..b274fb9 100644
> > --- a/fs/xfs/xfs_log.h
> > +++ b/fs/xfs/xfs_log.h
> > @@ -142,4 +142,7 @@ bool	xfs_log_in_recovery(struct xfs_mount *);
> >   
> >   xfs_lsn_t xlog_grant_push_threshold(struct xlog *log, int need_bytes);
> >   
> > +void xlog_use_incompat_feat(struct xlog *log);
> > +void xlog_drop_incompat_feat(struct xlog *log);
> > +
> >   #endif	/* __XFS_LOG_H__ */
> > diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> > index 4c41bbfa..c507041 100644
> > --- a/fs/xfs/xfs_log_priv.h
> > +++ b/fs/xfs/xfs_log_priv.h
> > @@ -449,6 +449,9 @@ struct xlog {
> >   	xfs_lsn_t		l_recovery_lsn;
> >   
> >   	uint32_t		l_iclog_roundoff;/* padding roundoff */
> > +
> > +	/* Users of log incompat features should take a read lock. */
> > +	struct rw_semaphore	l_incompat_users;
> >   };
> >   
> >   #define XLOG_BUF_CANCEL_BUCKET(log, blkno) \