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

[Mikulas Patocka]

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

Mikulas