block: take chunk_sectors into account in bio_split_write_zeroes

For zoned devices, write zeroes must be split at the zone boundary
which is represented as chunk_sectors.  For other uses like the
internally RAIDed NVMe devices it is probably at least useful.

Enhance get_max_io_size to know about write zeroes and use it in
bio_split_write_zeroes.  Also add a comment about the seemingly
nonsensical zero max_write_zeroes limit.

Fixes: 885fa13 ("block: implement splitting of REQ_OP_WRITE_ZEROES bios")
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Christoph Hellwig

block/blk-merge.c

@@ -171,17 +171,6 @@ struct bio *bio_split_discard(struct bio *bio, const struct queue_limits *lim,
 	return bio_submit_split(bio, split_sectors);
 }
 
-struct bio *bio_split_write_zeroes(struct bio *bio,
-		const struct queue_limits *lim, unsigned *nsegs)
-{
-	*nsegs = 0;
-	if (!lim->max_write_zeroes_sectors)
-		return bio;
-	if (bio_sectors(bio) <= lim->max_write_zeroes_sectors)
-		return bio;
-	return bio_submit_split(bio, lim->max_write_zeroes_sectors);
-}
-
 static inline unsigned int blk_boundary_sectors(const struct queue_limits *lim,
 						bool is_atomic)
 {
@@ -216,7 +205,9 @@ static inline unsigned get_max_io_size(struct bio *bio,
 	 * We ignore lim->max_sectors for atomic writes because it may less
 	 * than the actual bio size, which we cannot tolerate.
 	 */
-	if (is_atomic)
+	if (bio_op(bio) == REQ_OP_WRITE_ZEROES)
+		max_sectors = lim->max_write_zeroes_sectors;
+	else if (is_atomic)
 		max_sectors = lim->atomic_write_max_sectors;
 	else
 		max_sectors = lim->max_sectors;
@@ -403,6 +394,26 @@ struct bio *bio_split_zone_append(struct bio *bio,
 	return bio_submit_split(bio, split_sectors);
 }
 
+struct bio *bio_split_write_zeroes(struct bio *bio,
+		const struct queue_limits *lim, unsigned *nsegs)
+{
+	unsigned int max_sectors = get_max_io_size(bio, lim);
+
+	*nsegs = 0;
+
+	/*
+	 * An unset limit should normally not happen, as bio submission is keyed
+	 * off having a non-zero limit.  But SCSI can clear the limit in the
+	 * I/O completion handler, and we can race and see this.  Splitting to a
+	 * zero limit obviously doesn't make sense, so band-aid it here.
+	 */
+	if (!max_sectors)
+		return bio;
+	if (bio_sectors(bio) <= max_sectors)
+		return bio;
+	return bio_submit_split(bio, max_sectors);
+}
+
 /**
  * bio_split_to_limits - split a bio to fit the queue limits
  * @bio:     bio to be split