Hi,
This patchset brings multi-page bvec into block layer:
1) what is multi-page 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 multi-page bvec introduced?
Kent proposed the idea[1] first.
As system's RAM becomes much bigger than before, and 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.
As we saw, if CONFIG_THP_SWAP is enabled, BIO_MAX_PAGES can be configured
as much bigger, such as 512, which requires at least two 4K pages for holding
the bvec table.
With multi-page bvec:
- 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.
- 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].
- there is opportunity in future to improve memory footprint of bvecs.
3) how is multi-page bvec implemented in this patchset?
Patch 1 ~ 3 parpares for supporting multi-page bvec.
Patches 4 ~ 14 implement 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
- introduce bio_for_each_bvec() to iterate over multipage bvec for
splitting
bio and mapping sg
- keep current bio_for_each_segment*() to itereate over singlepage bvec
and
make sure current users won't be broken; especailly, convert to this
new helper prototype in single patch 21 given it is bascially a
mechanism
conversion
- deal with iomap & xfs's sub-pagesize io vec in patch 13
- enalbe multipage bvec in patch 14
Patch 15 redefines BIO_MAX_PAGES as 256.
Patch 16 documents usages of bio iterator helpers.
Patch 17~18 kills NO_SG_MERGE.
These patches can be found in the following git tree:
git: https://github.com/ming1/linux.git v5.0-blk_mp_bvec_v14
Lots of test(blktest, xfstests, ltp io, ...) have been run with this patchset,
and not see regression.
Thanks Christoph for reviewing 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.
Thanks Chritoph and Omar for reviewing V10/V11/V12, and provides lots of
helpful comments.
V14:
- drop patch(patch 4 in V13) for renaming bvec helpers, as suggested by
Jens
- use mp_bvec_* as multi-page bvec helper name
- fix one build issue, which is caused by missing one converion of
bio_for_each_segment_all in fs/gfs2
- fix one 32bit ARCH specific issue caused by segment boundary mask
overflow
V13:
- rebase on v5.0-rc2
- address Omar's comment on patch 1 of V12 by using V11's approach
- rename one local vairable in patch 15 as suggested by Christoph
V12:
- deal with non-cluster by max segment size & segment boundary limit
- rename bvec helper's name
- revert new change on bvec_iter_advance() in V11
- introduce rq_for_each_bvec()
- use simpler check on enalbing multi-page bvec
- fix Document change
V11:
- address most of reviews from Omar and christoph
- rename mp_bvec_* as segment_* helpers
- remove 'mp' parameter from bvec_iter_advance() and related helpers
- cleanup patch on bvec_split_segs() and blk_bio_segment_split(),
remove unnecessary checks
- simplify bvec_last_segment()
- drop bio_pages_all()
- introduce dedicated functions/file for handling non-cluser bio for
avoiding checking queue cluster before adding page to bio
- introduce bio_try_merge_segment() for simplifying iomap/xfs page
accounting code
- Fix Document change
V10:
- no any code change, just add more guys and list into patch's CC list,
as suggested by Christoph and Dave Chinner
V9:
- fix regression on iomap's sub-pagesize io vec, covered by patch 13
V8:
- remove prepare patches which all are merged to linus tree
