Re: [PATCH v2] block: make sure big bio is splitted into at most 256 bvecs

Eric Wheeler
2016-08-11T06:34:05Z

On Fri, 10 Jun 2016, Christoph Hellwig wrote:
> On Wed, 6 Apr 2016, Ming Lei wrote:
> >
> > 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:
> > [ 172.664813] [<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]
>
> The fixup to allow bio_clone support a larger size is the same one as to
> allow everyone else submitting larger bios: increase BIO_MAX_PAGES and
> create the required mempools to back that new larger size. Or just go
> for multipage biovecs..

Hi Christoph, Ming, everyone:

I'm hoping you can help me get this off of a list of stability fixes
related to changes around Linux 4.3. Ming's patch [1] is known to fix an
issue when a bio with bi_vcnt > BIO_MAX_PAGES is passed to
generic_make_request and later hits bio_clone. (Note that bi_vcnt can't
be trusted since immutable biovecs and needs to be re-counted unless you
own the bio, which Ming's patch does.)

The diffstat, 22 lines of which are commentary,
seems relatively minor and would land in stable for v4.3+:
block/blk-merge.c | 35 ++++++++++++++++++++++++++++++++---

I'm not sure I understood Christoph's suggestion; BIO_MAX_PAGES is a
static #define and we don't know what the the bi_vcnt from an arbitrary
driver might be. Wouldn't increasing BIO_MAX_PAGES just push the problem
further out into the future when bi_vcnt might again exceed BIO_MAX_PAGES?

Perhaps you could elaborate if I have misunderstood. Are you suggesting
that no driver should call generic_make_request when bi_vcnt > BIO_MAX_PAGES?

--
Eric Wheeler

[1] https://patchwork.kernel.org/patch/9169483/
Pasted below:

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@roesner-online.de>
Reported-by: Eric Wheeler <bcache@lists.ewheeler.net>
Cc: stable@vger.kernel.org (4.3+)
Cc: Shaohua Li <shli@fb.com>
Acked-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
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 c265348..839529b 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_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);