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

[Mikulas Patocka]

On Mon, 29 Aug 2016, Mike Snitzer wrote:

> On Sat, Aug 27 2016 at 11:09am -0400,
> 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
> 
> I'm left wondering whether it'd be better to revert commit dbba42d8a9e
> ("dm: eliminate unused "bioset" process for each bio-based DM device")

That would create one more process per dm device.

There is no need to create another process if the bio can be split in the 
device mapper.

> And also look for other areas of DM that could leverage the block core's
> relatively new splitting capabilities.
> 
> Otherwise, we'll just be sprinking more BIO_MAX_PAGES-based splitting
> code in DM targets because DM is so "special" that it doesn't need the
> block core's splitting (even though it clearly would now benefit).

It would be possible to split bios on BIO_MAX_PAGES boundary in the dm 
core. But I checked all the dm targets and I found problem only in 
dm-crypt and dm-log-writes. So there is no need to split bio bios for the 
other targets.

> Mike

Mikulas