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

[Ming Lei]

On Thu, Apr 7, 2016 at 9:56 AM, Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 7 Apr 2016, Ming Lei wrote:
>
>> On Thu, Apr 7, 2016 at 9:36 AM, Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx> 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:
>> >>
>> >> > [  172.660142] 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+)
>> >
>> > Ming Lei,
>> >
>> > get_max_io_size doesn't appear until 4.5 based on a quick LXR check so we
>> > won't see it in stable I don't think.
>> >
>> > It would be nice to see this fixed in 4.1 (if affected there).  Is there
>>
>> The issue should be introduced to v4.3 via 54efd50
>>
>> > another place this could be applied to be a bit more backward compatible?
>>
>> The v1 needn't change to get_max_io_size(), and it should be simple enough
>> to backport to previous stables, please try it:
>>
>> http://marc.info/?l=linux-block&m=145991422422927&w=2
>
> V1 changes blk_bio_segment_split() which doesn't exist until 4.3.
>
> How might you port this to v4.1.y?

Can you see the issue with v4.1?

You mentioned there are three reports:

> [2014-02-04] https://bugzilla.redhat.com/show_bug.cgi?id=1061339
> [2016-01-13] http://www.spinics.net/lists/linux-bcache/msg03335.html
             https://bugzilla.kernel.org/show_bug.cgi?id=110771
> [2016-03-25] http://thread.gmane.org/gmane.linux.kernel.bcache.devel/3607 [this thread]
             https://bugzilla.kernel.org/show_bug.cgi?id=114871

The first one has been fixed by '8423ae3 block: Fix cloning of
discard/write same bios', as mentioned in bugzilla.

The other two are reported on v4.4 and v4.5.

If you look at the patch description, it is just needed for 4.3+.

Or I am wrong?


Thanks,

>
> --
> Eric Wheeler
>
>
>>
>> Thanks,
>>
>> >
>> >> 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
>> >>
>> >>  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
>> >>
>> >>
>> > --
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