[dm-devel] dm-crypt: Fix error with too large bios

[Ming Lei]

On Sat, Aug 27, 2016 at 11:09 PM, Mikulas Patocka <mpatocka at redhat.com> wrote:
>
>
> On Fri, 26 Aug 2016, Mike Snitzer wrote:
>
>> On Thu, Aug 25 2016 at  4:13pm -0400,
>> Jens Axboe <axboe at kernel.dk> wrote:
>>
>> > On 08/25/2016 12:34 PM, Mikulas Patocka wrote:
>> > >
>> > >Device mapper can't split the bio in generic_make_request - it frees the
>> > >md->queue->bio_split bioset, to save one kernel thread per device. Device
>> > >mapper uses its own splitting mechanism.
>> > >
>> > >So what is the final decision? - should device mapper split the big bio or
>> > >should bcache not submit big bios?
>> > >
>> > >I think splitting big bios in the device mapper is better - simply because
>> > >it is much less code than reworking bcache to split bios internally.
>> > >
>> > >BTW. In the device mapper, we have a layer dm-io, that was created to work
>> > >around bio size limitations - it accepts unlimited I/O request and splits
>> > >it to several bios. When bio size limitations are gone, we could simplify
>> > >dm-io too.
>> >
>> > The patch from Ming Lei was applied for 4.8 the other day.
>>
>> See linux-block commit:
>> 4d70dca4eadf2f block: make sure a big bio is split into at most 256 bvecs
>> http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=4d70dca4eadf2f95abe389116ac02b8439c2d16c
>
> But this patch won't work for device mapper, blk_bio_segment_split is
> called from blk_queue_split and device mapper doesn't use blk_queue_split
> (it can't because it frees queue->bio_split).
>
> We still need these two patches:
> https://www.redhat.com/archives/dm-devel/2016-May/msg00211.html
> https://www.redhat.com/archives/dm-devel/2016-May/msg00210.html

About the 2nd patch, it might not be good enough because in theory
a small size bio still may include big bvecs, such as, each bvec points
to 512byte buffer, so strictly speaking the bvec number should
be checked instead of bio size.


-- 
Ming Lei