Re: [PATCH v1] block: make sure big bio is splitted into at most 256 bvecs

Kent Overstreet
2016-04-06T03:46:36Z

On Wed, Apr 06, 2016 at 11:43:32AM +0800, 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:
>
> > [ 172.660142] 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]
>
> 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>
> Cc: Kent Overstreet <kent.overstreet@gmail.com>
> Signed-off-by: Ming Lei <ming.lei@canonical.com>

That'll work

Acked-by: Kent Overstreet <kent.overstreet@gmail.com>

> ---
> V1:
> - Kent pointed out that using max io size can't cover
> the case of non-full bvecs/pages
>
> The issue can be reproduced by the following approach:
> - create one raid1 over two virtio-blk
> - build bcache device over the above raid1 and another cache device
> and bucket size is set 2Mbytes
> - set cache mode as writeback
> - run random write over ext4 on the bcache device
> - then the crash can be triggered
>
> block/blk-merge.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 2613531..7b96471 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -94,8 +94,10 @@ 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;
>
> bio_for_each_segment(bv, bio, iter) {
> + bvecs++;
> /*
> * If the queue doesn't support SG gaps and adding this
> * offset would create a gap, disallow it.
> @@ -103,6 +105,23 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
> goto split;
>
> + /*
> + * 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.
> + *
> + * TODO: deal with bio bounce's bio_clone() gracefully
> + * and convert the global limit into per-queue limit.
> + */
> + if (bvecs >= BIO_MAX_PAGES)
> + goto split;
> +
> if (sectors + (bv.bv_len >> 9) > max_sectors) {
> /*
> * Consider this a new segment if we're splitting in
> --
> 1.9.1
>