@@ -85,7 +85,8 @@ static inline unsigned get_max_io_size(struct request_queue *q,
static struct bio *blk_bio_segment_split(struct request_queue *q,
struct bio *bio,
struct bio_set *bs,
- unsigned *segs)
+ unsigned *segs,
+ bool *no_merge)
{
struct bio_vec bv, bvprv, *bvprvp = NULL;
struct bvec_iter iter;
@@ -94,9 +95,34 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
bool do_split = true;
struct bio *new = NULL;
const unsigned max_sectors = get_max_io_size(q, bio);
+ unsigned bvecs = 0;
+
+ *no_merge = true;
bio_for_each_segment(bv, bio, iter) {
/*
+ * With arbitrary bio size, the incoming bio may be very
+ * big. We have to split the bio into small bios so that
+ * each holds at most BIO_MAX_PAGES bvecs because
+ * bio_clone() can fail to allocate big bvecs.
+ *
+ * It should have been better to apply the limit per
+ * request queue in which bio_clone() is involved,
+ * instead of globally. The biggest blocker is
+ * bio_clone() in bio bounce.
+ *
+ * If bio is splitted by this reason, we should allow
+ * to continue bios merging.
+ *
+ * TODO: deal with bio bounce's bio_clone() gracefully
+ * and convert the global limit into per-queue limit.
+ */
+ if (bvecs++ >= BIO_MAX_PAGES) {
+ *no_merge = false;
+ goto split;
+ }
+
+ /*
* If the queue doesn't support SG gaps and adding this
* offset would create a gap, disallow it.
*/
@@ -171,13 +197,15 @@ void blk_queue_split(struct request_queue *q, struct bio **bio,
{
struct bio *split, *res;
unsigned nsegs;
+ bool no_merge_for_split = true;
if (bio_op(*bio) == REQ_OP_DISCARD)
split = blk_bio_discard_split(q, *bio, bs, &nsegs);
else if (bio_op(*bio) == REQ_OP_WRITE_SAME)
split = blk_bio_write_same_split(q, *bio, bs, &nsegs);
else
- split = blk_bio_segment_split(q, *bio, q->bio_split, &nsegs);
+ split = blk_bio_segment_split(q, *bio, q->bio_split, &nsegs,
+ &no_merge_for_split);
/* physical segments can be figured out during splitting */
res = split ? split : *bio;
@@ -186,7 +214,8 @@ void blk_queue_split(struct request_queue *q, struct bio **bio,
if (split) {
/* there isn't chance to merge the splitted bio */
- split->bi_rw |= REQ_NOMERGE;
+ if (no_merge_for_split)
+ split->bi_rw |= REQ_NOMERGE;
bio_chain(split, *bio);
trace_block_split(q, split, (*bio)->bi_iter.bi_sector);