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

Darrick J. Wong
2018-11-28T16:23:13Z

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