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-kernel...@roesner-online.de> Reported-by: Eric Wheeler <bca...@lists.ewheeler.net> Cc: sta...@vger.kernel.org (4.3+) Cc: Shaohua Li <s...@fb.com> Acked-by: Kent Overstreet <kent.overstr...@gmail.com> Signed-off-by: Ming Lei <ming....@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);
