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

[Mikulas Patocka]

On Mon, 29 Aug 2016, Ming Lei wrote:

> 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

This is not a problem.

dm-crypt allocates new pages for the bio and copies (and encrypts) the 
data from the original location to the new pages.

So yes, the original bio may have a long vector with many small fragments, 
but the newly allocated memory will be allocated in full pages and the 
outgoing bio's vector will have full pages.

Mikulas