Re: [PATCH] xfs: debug mode sysfs flag to force [un]pin the log tail

From: Brian Foster <bfoster@redhat.com>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH] xfs: debug mode sysfs flag to force [un]pin the log tail
Date: Wed, 21 Jun 2017 06:16:54 -0400	[thread overview]
Message-ID: <20170621101653.GD28914@bfoster.bfoster> (raw)
In-Reply-To: <20170620214832.GE4733@birch.djwong.org>

On Tue, Jun 20, 2017 at 02:48:32PM -0700, Darrick J. Wong wrote:
> On Fri, Jun 16, 2017 at 12:44:33PM -0400, Brian Foster wrote:
> > Create a debug mode only sysfs option to force pin the tail of the
> > log. This option can be used by test infrastructure to induce head
> > behind tail conditions. Specifically, this is intended to be used by
> > xfstests to reproduce log recovery problems after failed/corrupted
> > log writes overwrite the last good tail LSN in the log.
> > 
> > When enabled, AIL push attempts see every log item on the AIL in the
> > pinned state. This stalls metadata writeback and thus prevents the
> > current tail of the log from moving forward. When disabled,
> > subsequent AIL pushes observe the log items in their appropriate
> > state and filesystem operation continues as normal.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> 
> Looks ok enough to test...
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> 

I assume you mean Reviewed-by... thanks! :)

Brian

> --D
> 
> > ---
> > 
> > Hi all,
> > 
> > This patch is a supporting patch for an xfstests test I'm about to post
> > that pins the tail of the log in order to reproduce the log recovery
> > problem that appears to be the root of the problem in this[1] thread.
> > That is the primary motivation for the patch and so should probably be
> > reviewed with that context. IOW, if there's a better way to reproduce
> > the problem in the test without the need for kernel support, I'm happy
> > to drop this. Thoughts, reviews, flames appreciated.
> > 
> > Brian
> > 
> > [1] http://www.spinics.net/lists/linux-xfs/msg07499.html
> > 
> >  fs/xfs/xfs_log_priv.h  |  2 ++
> >  fs/xfs/xfs_sysfs.c     | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> >  fs/xfs/xfs_trans_ail.c | 20 +++++++++++++++++++-
> >  3 files changed, 67 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> > index c2604a5..bfbfde12 100644
> > --- a/fs/xfs/xfs_log_priv.h
> > +++ b/fs/xfs/xfs_log_priv.h
> > @@ -413,6 +413,8 @@ struct xlog {
> >  	void			*l_iclog_bak[XLOG_MAX_ICLOGS];
> >  	/* log record crc error injection factor */
> >  	uint32_t		l_badcrc_factor;
> > +	/* force pin the log tail */
> > +	bool			l_pin_tail;
> >  #endif
> >  	/* log recovery lsn tracking (for buffer submission */
> >  	xfs_lsn_t		l_recovery_lsn;
> > diff --git a/fs/xfs/xfs_sysfs.c b/fs/xfs/xfs_sysfs.c
> > index ec6e0e2..b86148a 100644
> > --- a/fs/xfs/xfs_sysfs.c
> > +++ b/fs/xfs/xfs_sysfs.c
> > @@ -378,6 +378,51 @@ log_badcrc_factor_show(
> >  }
> >  
> >  XFS_SYSFS_ATTR_RW(log_badcrc_factor);
> > +
> > +/*
> > + * DEBUG mode flag to force pin the tail of the log. Used from test
> > + * infrastructure to manufacture head-behind-tail conditions. DO NOT USE
> > + * DIRECTLY. This will lock up the fs!
> > + *
> > + * When this option is enabled, all log items present in the AIL are emulated as
> > + * being in the pinned state until the option is disabled. Once disabled, log
> > + * items return to their natural state and fs operation continues as normal.
> > + */
> > +STATIC ssize_t
> > +log_pin_tail_store(
> > +	struct kobject		*kobject,
> > +	const char		*buf,
> > +	size_t			count)
> > +{
> > +	struct xlog		*log = to_xlog(kobject);
> > +	int			ret;
> > +	int			val;
> > +
> > +	ret = kstrtoint(buf, 0, &val);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (val == 1)
> > +		log->l_pin_tail = true;
> > +	else if (val == 0)
> > +		log->l_pin_tail = false;
> > +	else
> > +		return -EINVAL;
> > +
> > +	return count;
> > +}
> > +
> > +STATIC ssize_t
> > +log_pin_tail_show(
> > +	struct kobject		*kobject,
> > +	char			*buf)
> > +{
> > +	struct xlog		*log = to_xlog(kobject);
> > +
> > +	return snprintf(buf, PAGE_SIZE, "%d\n", log->l_pin_tail ? 1 : 0);
> > +}
> > +XFS_SYSFS_ATTR_RW(log_pin_tail);
> > +
> >  #endif	/* DEBUG */
> >  
> >  static struct attribute *xfs_log_attrs[] = {
> > @@ -387,6 +432,7 @@ static struct attribute *xfs_log_attrs[] = {
> >  	ATTR_LIST(write_grant_head),
> >  #ifdef DEBUG
> >  	ATTR_LIST(log_badcrc_factor),
> > +	ATTR_LIST(log_pin_tail),
> >  #endif
> >  	NULL,
> >  };
> > diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
> > index 9056c0f..c901e61 100644
> > --- a/fs/xfs/xfs_trans_ail.c
> > +++ b/fs/xfs/xfs_trans_ail.c
> > @@ -27,6 +27,7 @@
> >  #include "xfs_trace.h"
> >  #include "xfs_error.h"
> >  #include "xfs_log.h"
> > +#include "xfs_log_priv.h"
> >  
> >  #ifdef DEBUG
> >  /*
> > @@ -325,6 +326,23 @@ xfs_ail_delete(
> >  	xfs_trans_ail_cursor_clear(ailp, lip);
> >  }
> >  
> > +static inline uint
> > +xfsaild_push_item(
> > +	struct xfs_ail		*ailp,
> > +	struct xfs_log_item	*lip)
> > +{
> > +#ifdef DEBUG
> > +	/*
> > +	 * If tail pinning is enabled, skip the push and track all items as
> > +	 * pinned to force pin the log tail. This helps induce head-behind-tail
> > +	 * conditions.
> > +	 */
> > +	if (ailp->xa_mount->m_log->l_pin_tail)
> > +		return XFS_ITEM_PINNED;
> > +#endif
> > +	return lip->li_ops->iop_push(lip, &ailp->xa_buf_list);
> > +}
> > +
> >  static long
> >  xfsaild_push(
> >  	struct xfs_ail		*ailp)
> > @@ -382,7 +400,7 @@ xfsaild_push(
> >  		 * rely on the AIL cursor implementation to be able to deal with
> >  		 * the dropped lock.
> >  		 */
> > -		lock_result = lip->li_ops->iop_push(lip, &ailp->xa_buf_list);
> > +		lock_result = xfsaild_push_item(ailp, lip);
> >  		switch (lock_result) {
> >  		case XFS_ITEM_SUCCESS:
> >  			XFS_STATS_INC(mp, xs_push_ail_success);
> > -- 
> > 2.7.5
> > 
> > --
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