Subject: Re: [PATCH] xfs: introduce "metasync" api to sync metadata to fsblock

From: Pingfan Liu <[email-masked]>
Date: Mon, 14 Oct 2019 16:33:15 +0800

On Sun, Oct 13, 2019 at 09:34:17AM -0700, Darrick J. Wong wrote:
> On Sun, Oct 13, 2019 at 10:37:00PM +0800, Pingfan Liu wrote:
> > When using fadump (fireware assist dump) mode on powerpc, a mismatch
> > between grub xfs driver and kernel xfs driver has been obsevered.  Note:
> > fadump boots up in the following sequence: fireware -> grub reads kernel
> > and initramfs -> kernel boots.
> > 
> > The process to reproduce this mismatch:
> >   - On powerpc, boot kernel with fadump=on and edit /etc/kdump.conf.
> >   - Replacing "path /var/crash" with "path /var/crashnew", then, "kdumpctl
> >     restart" to rebuild the initramfs. Detail about the rebuilding looks
> >     like: mkdumprd /boot/initramfs-`uname -r`.img.tmp;
> >           mv /boot/initramfs-`uname -r`.img.tmp /boot/initramfs-`uname -r`.img
> >           sync
> >   - "echo c >/proc/sysrq-trigger".
> > 
> > The result:
> > The dump image will not be saved under /var/crashnew/* as expected, but
> > still saved under /var/crash.
> > 
> > The root cause:
> > As Eric pointed out that on xfs, 'sync' ensures the consistency by writing
> > back metadata to xlog, but not necessary to fsblock. This raises issue if
> > grub can not replay the xlog before accessing the xfs files. Since the
> > above dir entry of initramfs should be saved as inline data with xfs_inode,
> > so xfs_fs_sync_fs() does not guarantee it written to fsblock.
> > 
> > umount can be used to write metadata fsblock, but the filesystem can not be
> > umounted if still in use.
> > 
> > There are two ways to fix this mismatch, either grub or xfs. It may be
> > easier to do this in xfs side by introducing an interface to flush metadata
> > to fsblock explicitly.
> > 
> > With this patch, metadata can be written to fsblock by:
> >   # update AIL
> >   sync
> >   # new introduced interface to flush metadata to fsblock
> >   mount -o remount,metasync mountpoint
> 
> I think this ought to be an ioctl or some sort of generic call since the
> jbd2 filesystems (ext3, ext4, ocfs2) suffer from the same "$BOOTLOADER
> is too dumb to recover logs but still wants to write to the fs"
> checkpointing problem.
Yes, a syscall sounds more reasonable.
> 
> (Or maybe we should just put all that stuff in a vfat filesystem, I
> don't know...)
I think it is unavoidable to involve in each fs' implementation. What
about introducing an interface sync_to_fsblock(struct super_block *sb) in
the struct super_operations, then let each fs manage its own case?
> 
> --D
> 
> > Signed-off-by: Pingfan Liu <[email-masked]>
> > Cc: "Darrick J. Wong" <[email-masked]>
> > Cc: Dave Chinner <[email-masked]>
> > Cc: Eric Sandeen <[email-masked]>
> > Cc: Hari Bathini <[email-masked]>
> > Cc: [email-masked]
> > To: [email-masked]
> > ---
> >  fs/xfs/xfs_mount.h      |  1 +
> >  fs/xfs/xfs_super.c      | 15 ++++++++++++++-
> >  fs/xfs/xfs_trans.h      |  2 ++
> >  fs/xfs/xfs_trans_ail.c  | 26 +++++++++++++++++++++++++-
> >  fs/xfs/xfs_trans_priv.h |  1 +
> >  5 files changed, 43 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> > index fdb60e0..85f32e6 100644
> > --- a/fs/xfs/xfs_mount.h
> > +++ b/fs/xfs/xfs_mount.h
> > @@ -243,6 +243,7 @@ typedef struct xfs_mount {
> >  #define XFS_MOUNT_FILESTREAMS	(1ULL << 24)	/* enable the filestreams
> >  						   allocator */
> >  #define XFS_MOUNT_NOATTR2	(1ULL << 25)	/* disable use of attr2 format */
> > +#define XFS_MOUNT_METASYNC	(1ull << 26)	/* write meta to fsblock */
> >  
> >  #define XFS_MOUNT_DAX		(1ULL << 62)	/* TEST ONLY! */
> >  
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index 8d1df9f..41df810 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -59,7 +59,7 @@ enum {
> >  	Opt_filestreams, Opt_quota, Opt_noquota, Opt_usrquota, Opt_grpquota,
> >  	Opt_prjquota, Opt_uquota, Opt_gquota, Opt_pquota,
> >  	Opt_uqnoenforce, Opt_gqnoenforce, Opt_pqnoenforce, Opt_qnoenforce,
> > -	Opt_discard, Opt_nodiscard, Opt_dax, Opt_err,
> > +	Opt_discard, Opt_nodiscard, Opt_dax, Opt_metasync, Opt_err
> >  };
> >  
> >  static const match_table_t tokens = {
> > @@ -106,6 +106,7 @@ static const match_table_t tokens = {
> >  	{Opt_discard,	"discard"},	/* Discard unused blocks */
> >  	{Opt_nodiscard,	"nodiscard"},	/* Do not discard unused blocks */
> >  	{Opt_dax,	"dax"},		/* Enable direct access to bdev pages */
> > +	{Opt_metasync,	"metasync"},	/* one shot to write meta to fsblock */
> >  	{Opt_err,	NULL},
> >  };
> >  
> > @@ -338,6 +339,9 @@ xfs_parseargs(
> >  			mp->m_flags |= XFS_MOUNT_DAX;
> >  			break;
> >  #endif
> > +		case Opt_metasync:
> > +			mp->m_flags |= XFS_MOUNT_METASYNC;
> > +			break;
> >  		default:
> >  			xfs_warn(mp, "unknown mount option [%s].", p);
> >  			return -EINVAL;
> > @@ -1259,6 +1263,9 @@ xfs_fs_remount(
> >  			mp->m_flags |= XFS_MOUNT_SMALL_INUMS;
> >  			mp->m_maxagi = xfs_set_inode_alloc(mp, sbp->sb_agcount);
> >  			break;
> > +		case Opt_metasync:
> > +			mp->m_flags |= XFS_MOUNT_METASYNC;
> > +			break;
> >  		default:
> >  			/*
> >  			 * Logically we would return an error here to prevent
> > @@ -1286,6 +1293,12 @@ xfs_fs_remount(
> >  		}
> >  	}
> >  
> > +	if (mp->m_flags & XFS_MOUNT_METASYNC) {
> > +		xfs_ail_push_sync(mp->m_ail);
> > +		/* one shot flag */
> > +		mp->m_flags &= ~XFS_MOUNT_METASYNC;
> 
> Wait, so the mount flag magically disables itself?
> 
> This really sounds like a system call, not the kinds of long term
> behavioral modifications that mount options are for.
> 
Yeah, this one-shot behavior is not suitable for mount.
> > +	}
> > +
> >  	/* ro -> rw */
> >  	if ((mp->m_flags & XFS_MOUNT_RDONLY) && !(*flags & SB_RDONLY)) {
> >  		if (mp->m_flags & XFS_MOUNT_NORECOVERY) {
> > diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> > index 64d7f17..fcdb902 100644
> > --- a/fs/xfs/xfs_trans.h
> > +++ b/fs/xfs/xfs_trans.h
> > @@ -242,6 +242,8 @@ void		xfs_trans_buf_set_type(struct xfs_trans *, struct xfs_buf *,
> >  void		xfs_trans_buf_copy_type(struct xfs_buf *dst_bp,
> >  					struct xfs_buf *src_bp);
> >  
> > +void		xfs_ail_push_sync(struct xfs_ail *ailp);
> > +
> >  extern kmem_zone_t	*xfs_trans_zone;
> >  
> >  #endif	/* __XFS_TRANS_H__ */
> > diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
> > index 6ccfd75..b8d8df1 100644
> > --- a/fs/xfs/xfs_trans_ail.c
> > +++ b/fs/xfs/xfs_trans_ail.c
> > @@ -488,7 +488,11 @@ xfsaild_push(
> >  	xfs_trans_ail_cursor_done(&cur);
> >  	spin_unlock(&ailp->ail_lock);
> >  
> > -	if (xfs_buf_delwri_submit_nowait(&ailp->ail_buf_list))
> > +	if (unlikely(mp->m_flags & XFS_MOUNT_METASYNC)) {
> > +		xfs_buf_delwri_submit(&ailp->ail_buf_list);
> 
> I guess this never fails, because write IO is 100% successful?
No, unfortunately, xfs_buf_delwri_submit() can return error. Do you
suggest to pass the error to the sync call?

Thanks for your kindly review.

Regards,
	Pingfan

[Keep the following original content unchanged. And cc Jan Kara, linux-fsdevel due to this
design may touch the vfs layer]
> 
> --D
> 
> > +		ailp->ail_log_flush++;
> > +		wake_up_all(&ailp->pushed_que);
> > +	} else if (xfs_buf_delwri_submit_nowait(&ailp->ail_buf_list))
> >  		ailp->ail_log_flush++;
> >  
> >  	if (!count || XFS_LSN_CMP(lsn, target) >= 0) {
> > @@ -641,6 +645,25 @@ xfs_ail_push(
> >  	wake_up_process(ailp->ail_task);
> >  }
> >  
> > +void
> > +xfs_ail_push_sync(
> > +	struct xfs_ail		*ailp)
> > +{
> > +	xfs_lsn_t		sync_lsn;
> > +	DEFINE_WAIT(wait);
> > +
> > +	sync_lsn = xfs_ail_max_lsn(ailp);
> > +	for (;;) {
> > +		xfs_ail_push(ailp, sync_lsn);
> > +		prepare_to_wait(&ailp->pushed_que, &wait, TASK_INTERRUPTIBLE);
> > +		if (XFS_LSN_CMP(READ_ONCE(ailp->ail_target_prev),
> > +			sync_lsn) >= 0)
> > +			break;
> > +		schedule();
> > +	}
> > +	finish_wait(&ailp->pushed_que, &wait);
> > +}
> > +
> >  /*
> >   * Push out all items in the AIL immediately
> >   */
> > @@ -834,6 +857,7 @@ xfs_trans_ail_init(
> >  	spin_lock_init(&ailp->ail_lock);
> >  	INIT_LIST_HEAD(&ailp->ail_buf_list);
> >  	init_waitqueue_head(&ailp->ail_empty);
> > +	init_waitqueue_head(&ailp->pushed_que);
> >  
> >  	ailp->ail_task = kthread_run(xfsaild, ailp, "xfsaild/%s",
> >  			ailp->ail_mount->m_fsname);
> > diff --git a/fs/xfs/xfs_trans_priv.h b/fs/xfs/xfs_trans_priv.h
> > index 2e073c1..9fe3cc6 100644
> > --- a/fs/xfs/xfs_trans_priv.h
> > +++ b/fs/xfs/xfs_trans_priv.h
> > @@ -61,6 +61,7 @@ struct xfs_ail {
> >  	int			ail_log_flush;
> >  	struct list_head	ail_buf_list;
> >  	wait_queue_head_t	ail_empty;
> > +	wait_queue_head_t	pushed_que;
> >  };
> >  
> >  /*
> > -- 
> > 2.7.5
> >