linux-kernel - [PATCH v3] block: make sure big bio is splitted into at most 256 bvecs

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Date:	Mon, 15 Aug 2016 23:11:22 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org
Cc:	linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Eric Wheeler <bcache@...ts.ewheeler.net>,
	Sebastian Roesner <sroesner-kernelorg@...sner-online.de>,
	Ming Lei <ming.lei@...onical.com>,
	stable@...r.kernel.org (4.3+), Shaohua Li <shli@...com>,
	Jens Axboe <axboe@...nel.dk>
Subject: [PATCH v3] block: make sure big bio is splitted into at most 256 bvecs

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@...sner-online.de>
Reported-by: Eric Wheeler <bcache@...ts.ewheeler.net>
Cc: stable@...r.kernel.org (4.3+)
Cc: Shaohua Li <shli@...com>
Acked-by: Kent Overstreet <kent.overstreet@...il.com>
Signed-off-by: Ming Lei <ming.lei@...onical.com>
---
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