After arbitrary bio size is supported, 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 for safety reason, such
as bio_clone().
This patch fixes the following kernel crash:
> [ 172.660142] BUG: unable to handle kernel NULL pointer dereference at 0000000000000028
> [ 172.660229] IP: [<ffffffff811e53b4>] bio_trim+0xf/0x2a
> [ 172.660289] PGD 7faf3e067 PUD 7f9279067 PMD 0
> [ 172.660399] Oops: 0000 [#1] SMP
> [...]
> [ 172.664780] Call Trace:
> [ 172.664813] [<ffffffffa007f3be>] ? raid1_make_request+0x2e8/0xad7 [raid1]
> [ 172.664846] [<ffffffff811f07da>] ? blk_queue_split+0x377/0x3d4
> [ 172.664880] [<ffffffffa005fb5f>] ? md_make_request+0xf6/0x1e9 [md_mod]
> [ 172.664912] [<ffffffff811eb860>] ? generic_make_request+0xb5/0x155
> [ 172.664947] [<ffffffffa0445c89>] ? prio_io+0x85/0x95 [bcache]
> [ 172.664981] [<ffffffffa0448252>] ? register_cache_set+0x355/0x8d0 [bcache]
> [ 172.665016] [<ffffffffa04497d3>] ? register_bcache+0x1006/0x1174 [bcache]
The issue can be reproduced by the following steps:
- create one raid1 over two virtio-blk
- build bcache device over the above raid1 and another cache device
and bucket size is set as 2Mbytes
- set cache mode as writeback
- run random write over ext4 on the bcache device
Fixes: 54efd50(block: make generic_make_request handle arbitrarily sized bios)
Reported-by: Sebastian Roesner <sroesner-kernelorg@xxxxxxxxxxxxxxxxx>
Reported-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx (4.3+)
Cc: Shaohua Li <shli@xxxxxx>
Acked-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
---
V3:
- rebase against v4.8-rc1 since .bi_rw of bio is renamed
as .bi_opf
V2:
- don't mark as REQ_NOMERGE in case the bio is splitted
for reaching the limit of bvecs count
V1:
- Kent pointed out that using max io size can't cover
the case of non-full bvecs/pages
block/blk-merge.c | 35 ++++++++++++++++++++++++++++++++---
1 file changed, 32 insertions(+), 3 deletions(-)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 3eec75a..c44d3e9 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -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_opf |= REQ_NOMERGE;
+ if (no_merge_for_split)
+ split->bi_opf |= REQ_NOMERGE;
bio_chain(split, *bio);
trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
--
2.7.4
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