Re: [PATCH v1 5/7] xfs: Add device retry

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Nov 28, 2018 at 04:08:50PM +1100, Dave Chinner wrote:
> On Tue, Nov 27, 2018 at 08:49:49PM -0700, Allison Henderson wrote:
> > Check to see if the _xfs_buf_read fails.  If so loop over the
> > available mirrors and retry the read
> > 
> > Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
> > ---
> >  fs/xfs/xfs_buf.c | 28 +++++++++++++++++++++++++++-
> >  1 file changed, 27 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index dd8ba59..f102d01 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -21,6 +21,7 @@
> >  #include <linux/migrate.h>
> >  #include <linux/backing-dev.h>
> >  #include <linux/freezer.h>
> > +#include <linux/blkdev.h>
> >  
> >  #include "xfs_format.h"
> >  #include "xfs_log_format.h"
> > @@ -808,6 +809,8 @@ xfs_buf_read_map(
> >  	const struct xfs_buf_ops *ops)
> >  {
> >  	struct xfs_buf		*bp;
> > +	struct request_queue	*q;
> > +	unsigned short		i;
> >  
> >  	flags |= XBF_READ;
> >  
> > @@ -820,7 +823,30 @@ xfs_buf_read_map(
> >  	if (!(bp->b_flags & XBF_DONE)) {
> >  		XFS_STATS_INC(target->bt_mount, xb_get_read);
> >  		bp->b_ops = ops;
> > -		_xfs_buf_read(bp, flags);
> > +		q = bdev_get_queue(bp->b_target->bt_bdev);
> > +
> > +		/*
> > +		 * Mirrors are indexed 1 - n, specified through the rw_hint.
> > +		 * Setting the hint to 0 is unspecified and allows the block
> > +		 * layer to decide.
> > +		 */
> > +		for (i = 0; i <= blk_queue_get_mirrors(q); i++) {
> > +			bp->b_error = 0;
> > +			bp->b_rw_hint = i;
> > +			_xfs_buf_read(bp, flags);
> 
> So the first time through this loop the block layer devices what
> device to read from, then we iterate devices 1..n on error.
> 
> Whihc means if device 0 is the only one with good information in it,
> we may not ever actually read from it.
> 
> I'd suggest that a hint of "-1" (or equivalent max value) should be
> used for "device selects mirror leg" rather than 0, so we can
> actually read from the first device on command.

"read from the first device on command" => "set bio.bi_rw_hint = 1"...

> i.e.
> 		bp->b_error = 0;
> 		bp->b_rw_hint = -1;

...which is confusing.  The intended behavior for this RFC (though not
so well documented) is that bi_rw_hint == 0 means "let the device
choose", and rw_hint > 1 means "choose mirror (rw_hint - 1)".  That's
sort of an odd behavior because now we have:

blk_queue_get_mirrors(q) returns 5 (as in 5 mirrors) but we access the
5 mirrors as indices 1-5, not 0-4 like most programmers would probably
expect.

Also, I think it's probably necessary to create a #define to attach a
name to the "let the device choose" value...

#define BIO_RW_HINT_ANY_MIRROR	(0)

for (i = BIO_RW_HINT_ANY_MIRROR; i <= blk_queue_get_mirrors(q); i++) {
	...
	bp->b_rw_hint = i;
	...
	_xfs_buf_read(bp, flags);
	...
}

(or offset things -1 like you propose)

--D

> 		_xfs_buf_read(bp, flags);
> 
> 		if (!bp->b_error)
> 			return bp;
> 
> 		/* manual iteration to find a good copy */
> 		for (i = 0; i <= blk_queue_get_mirrors(q); i++) {
> 			bp->b_error = 0;
> 			bp->b_rw_hint = i;
> 			_xfs_buf_read(bp, flags);
> ......
> > +
> > +			switch (bp->b_error) {
> > +			case -EIO:
> > +			case -EFSCORRUPTED:
> > +			case -EFSBADCRC:
> > +				/* loop again */
> > +				continue;
> > +			default:
> > +				goto retry_done;
> 
> Just return bp here, don't need a jump label for it.
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com