On Mon, 29 Aug 2016, Ming Lei wrote: > On Sat, Aug 27, 2016 at 11:09 PM, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > > > > On Fri, 26 Aug 2016, Mike Snitzer wrote: > > > >> On Thu, Aug 25 2016 at 4:13pm -0400, > >> Jens Axboe <axboe@xxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
