On Wed, 6 Apr 2016, Ming Lei wrote: > On Wed, Apr 6, 2016 at 1:44 AM, Ming Lei <ming.lei@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> > > Reported-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> > > Cc: stable@xxxxxxxxxxxxxxx (4.2+) > > Cc: Shaohua Li <shli@xxxxxx> > > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx> > > --- > > I can reproduce the issue and verify the fix by the following approach: > > - create one raid1 over two virtio-blk > > - build bcache device over the above raid1 and another cache device. > > - set cache mode as writeback > > - run random write over ext4 on the bcache device > > - then the crash can be triggered > > For anyone who is interested in issue/fix, forget to mention: > > The bucket size should be set as bigger than 1M during making bcache. > In my test, the bucket size is 2M. Does the bucket size dictate the ideal cached data size, or is it just an optimization for erase block boundaries on the SSD? Are reads/writes smaller than the bucket size still cached effectively, or does a 2MB bucket slurp up 2MB of backing data along with it? For example, if 64k is our ideal IO size, should we use 64k buckets? -- Eric Wheeler > > Thanks, > Ming > > > > > block/blk-merge.c | 12 ++++++++++++ > > 1 file changed, 12 insertions(+) > > > > diff --git a/block/blk-merge.c b/block/blk-merge.c > > index 2613531..9a8651f 100644 > > --- a/block/blk-merge.c > > +++ b/block/blk-merge.c > > @@ -79,6 +79,18 @@ static inline unsigned get_max_io_size(struct request_queue *q, > > /* aligned to logical block size */ > > sectors &= ~(mask >> 9); > > > > + /* > > + * 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 for safety reason, such as > > + * bio_clone(). > > + * > > + * In the future, the limit might be converted into per-queue > > + * flag. > > + */ > > + sectors = min_t(unsigned, sectors, BIO_MAX_PAGES << > > + (PAGE_CACHE_SHIFT - 9)); > > + > > return sectors; > > } > > > > -- > > 1.9.1 > > > -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html