[PATCH v2 10/11] xfs_scrub: fix read verify disk error handling strategy

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

From: Darrick J. Wong <darrick.wong@oracle.com>

The error handling strategy for media errors is totally bogus.  First of
all, short reads are entirely unhandled -- when we encounter a short
read, we know the disk was able to feed us the beginning of what we
asked for, so we need to single-step through the remainder to try to
capture the exact error that we hit.

Second, an actual IO error causes the entire region to be marked bad
even though it could be just a few MB of a multi-gigabyte extent that's
bad.  Therefore, single-step each block in the IO request until we stop
getting IO errors to find out if all the blocks are bad or if it's just
that extent.

Third, fix the fact that the loop updates its own counter variables with
the length fed to read(), which doesn't necessarily have anything to do
with the amount of data that the read actually produced.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
v2: fix errors_seen bogosity
---
 scrub/read_verify.c |   86 ++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 74 insertions(+), 12 deletions(-)

diff --git a/scrub/read_verify.c b/scrub/read_verify.c
index 231df802..8aec25de 100644
--- a/scrub/read_verify.c
+++ b/scrub/read_verify.c
@@ -173,30 +173,92 @@ read_verify(
 	struct read_verify		*rv = arg;
 	struct read_verify_pool		*rvp;
 	unsigned long long		verified = 0;
+	ssize_t				io_max_size;
 	ssize_t				sz;
 	ssize_t				len;
+	int				io_error;
 	int				ret;
 
 	rvp = (struct read_verify_pool *)wq->wq_ctx;
+	if (rvp->runtime_error)
+		return;
+
+	io_max_size = RVP_IO_MAX_SIZE;
+
 	while (rv->io_length > 0) {
-		len = min(rv->io_length, RVP_IO_MAX_SIZE);
+		io_error = 0;
+		len = min(rv->io_length, io_max_size);
 		dbg_printf("diskverify %d %"PRIu64" %zu\n", rvp->disk->d_fd,
 				rv->io_start, len);
 		sz = disk_read_verify(rvp->disk, rvp->readbuf, rv->io_start,
 				len);
-		if (sz < 0) {
-			dbg_printf("IOERR %d %"PRIu64" %zu\n",
-					rvp->disk->d_fd, rv->io_start, len);
-			/* IO error, so try the next logical block. */
-			len = rvp->miniosz;
-			rvp->ioerr_fn(rvp->ctx, rvp->disk, rv->io_start, len,
-					errno, rv->io_end_arg);
+		if (sz == len && io_max_size < rvp->miniosz) {
+			/*
+			 * If the verify request was 100% successful and less
+			 * than a single block in length, we were trying to
+			 * read to the end of a block after a short read.  That
+			 * suggests there's something funny with this device,
+			 * so single-step our way through the rest of the @rv
+			 * range.
+			 */
+			io_max_size = rvp->miniosz;
+		} else if (sz < 0) {
+			io_error = errno;
+
+			/* Runtime error, bail out... */
+			if (io_error != EIO && io_error != EILSEQ) {
+				rvp->runtime_error = io_error;
+				return;
+			}
+
+			/*
+			 * A direct read encountered an error while performing
+			 * a multi-block read.  Reduce the transfer size to a
+			 * single block so that we can identify the exact range
+			 * of bad blocks and good blocks.  We single-step all
+			 * the way to the end of the @rv range, (re)starting
+			 * with the block that just failed.
+			 */
+			if (io_max_size > rvp->miniosz) {
+				io_max_size = rvp->miniosz;
+				continue;
+			}
+
+			/*
+			 * A direct read hit an error while we were stepping
+			 * through single blocks.  Mark everything bad from
+			 * io_start to the next miniosz block.
+			 */
+			sz = rvp->miniosz - (rv->io_start % rvp->miniosz);
+			dbg_printf("IOERR %d @ %"PRIu64" %zu err %d\n",
+					rvp->disk->d_fd, rv->io_start, sz,
+					io_error);
+			rvp->ioerr_fn(rvp->ctx, rvp->disk, rv->io_start, sz,
+					io_error, rv->io_end_arg);
+		} else if (sz < len) {
+			/*
+			 * A short direct read suggests that we might have hit
+			 * an IO error midway through the read but still had to
+			 * return the number of bytes that were actually read.
+			 *
+			 * We need to force an EIO, so try reading the rest of
+			 * the block (if it was a partial block read) or the
+			 * next full block.
+			 */
+			io_max_size = rvp->miniosz - (sz % rvp->miniosz);
+			dbg_printf("SHORT %d READ @ %"PRIu64" %zu try for %zd\n",
+					rvp->disk->d_fd, rv->io_start, sz,
+					io_max_size);
+		} else {
+			/* We should never get back more bytes than we asked. */
+			assert(sz == len);
 		}
 
-		progress_add(len);
-		verified += len;
-		rv->io_start += len;
-		rv->io_length -= len;
+		progress_add(sz);
+		if (io_error == 0)
+			verified += sz;
+		rv->io_start += sz;
+		rv->io_length -= sz;
 	}
 
 	free(rv);