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

Dave Chinner
2018-11-28T05:08:57Z

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.

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