On 06/26/2017 06:09 AM, Ming Lei wrote: > Hi, > > This patchset brings multipage bvec into block layer: > > 1) what is multipage bvec? > > Multipage bvecs means that one 'struct bio_bvec' can hold > multiple pages which are physically contiguous instead > of one single page used in linux kernel for long time. > > 2) why is multipage bvec introduced? > > Kent proposed the idea[1] first. > > As system's RAM becomes much bigger than before, and > at the same time huge page, transparent huge page and > memory compaction are widely used, it is a bit easy now > to see physically contiguous pages from fs in I/O. > On the other hand, from block layer's view, it isn't > necessary to store intermediate pages into bvec, and > it is enough to just store the physicallly contiguous > 'segment' in each io vector. > > Also huge pages are being brought to filesystem and swap > [2][6], we can do IO on a hugepage each time[3], which > requires that one bio can transfer at least one huge page > one time. Turns out it isn't flexiable to change BIO_MAX_PAGES > simply[3][5]. Multipage bvec can fit in this case very well. > > With multipage bvec: > > - segment handling in block layer can be improved much > in future since it should be quite easy to convert > multipage bvec into segment easily. For example, we might > just store segment in each bvec directly in future. > > - bio size can be increased and it should improve some > high-bandwidth IO case in theory[4]. > > - Inside block layer, both bio splitting and sg map can > become more efficient than before by just traversing the > physically contiguous 'segment' instead of each page. > > - there is opportunity in future to improve memory footprint > of bvecs. > > 3) how is multipage bvec implemented in this patchset? > > The 1st 18 patches comment on some special cases and deal with > some special cases of direct access to bvec table. > > The 2nd part(19~29) implements multipage bvec in block layer: > > - put all tricks into bvec/bio/rq iterators, and as far as > drivers and fs use these standard iterators, they are happy > with multipage bvec > > - use multipage bvec to split bio and map sg > > - bio_for_each_segment_all() changes > this helper pass pointer of each bvec directly to user, and > it has to be changed. Two new helpers(bio_for_each_segment_all_sp() > and bio_for_each_segment_all_mp()) are introduced. > > The 3rd part(30~49) convert current users of bio_for_each_segment_all() > to bio_for_each_segment_all_sp()/bio_for_each_segment_all_mp(). > > The last part(50~51) enables multipage bvec. > > These patches can be found in the following git tree: > > https://github.com/ming1/linux/commits/mp-bvec-1.4-v4.12-rc > > Thanks Christoph for looking at the early version and providing > very good suggestions, such as: introduce bio_init_with_vec_table(), > remove another unnecessary helpers for cleanup and so on. > > Any comments are welcome! I'll take some time to review this over the next week or so. In any case, it's a little late to stuff into 4.13 and get a decent amount of exposure and testing on it. A 4.14 target for this would be the way to go, imho. -- Jens Axboe
