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

[Allison Henderson <allison.henderson@oracle.com>]

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@kernel.org>
>>
>> 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@gmail.com>
Thank you!

Allison

> 
>> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
>> Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
>> Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
>> ---
>>   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) \
> 
>