[PATCH v3 0/5] io_uring/rsrc: coalescing multi-hugepage registered buffers

[Chenliang Li <cliang01.li@samsung.com>]

Registered buffers are stored and processed in the form of bvec array,
each bvec element typically points to a PAGE_SIZE page but can also work
with hugepages. Specifically, a buffer consisting of a hugepage is
coalesced to use only one hugepage bvec entry during registration.
This coalescing feature helps to save both the space and DMA-mapping time.

However, currently the coalescing feature doesn't work for multi-hugepage
buffers. For a buffer with several 2M hugepages, we still split it into
thousands of 4K page bvec entries while in fact, we can just use a
handful of hugepage bvecs.

This patch series enables coalescing registered buffers with more than
one hugepages. It optimizes the DMA-mapping time and saves memory for
these kind of buffers.

Perf diff of 8M(4*2M) hugepage fixed buffer fio test:

fio/t/io_uring -d64 -s32 -c32 -b8388608 -p0 -B1 -F0 -n1 -O1 -r10 \
-R1 /dev/nvme0n1

Before          After           Symbol

5.90%                           [k] __blk_rq_map_sg
3.70%                           [k] dma_direct_map_sg
3.07%                           [k] dma_pool_alloc
1.12%                           [k] sg_next
                +0.44%          [k] dma_map_page_attrs

First three patches prepare for adding the multi-hugepage coalescing
into buffer registration, the 4th patch enables the feature. The 5th
patch add test cases for this feature in liburing.

-----------------
Changes since v2:

- Modify the loop iterator increment to make code cleaner
- Minor fix to the return procedure in coalesced buffer account
- Correct commit messages
- Add test cases in liburing

v2 : https://lore.kernel.org/io-uring/20240513020149.492727-1-cliang01.li@samsung.com/T/#t

Changes since v1:

- Split into 4 patches
- Fix code style issues
- Rearrange the change of code for cleaner look
- Add speciallized pinned page accounting procedure for coalesced
  buffers
- Reordered the newly add fields in imu struct for better compaction

v1 : https://lore.kernel.org/io-uring/20240506075303.25630-1-cliang01.li@samsung.com/T/#u

Chenliang Li (5):
  io_uring/rsrc: add hugepage buffer coalesce helpers
  io_uring/rsrc: store folio shift and mask into imu
  io_uring/rsrc: add init and account functions for coalesced imus
  io_uring/rsrc: enable multi-hugepage buffer coalescing
  liburing: add test cases for hugepage registered buffers

 io_uring/rsrc.c | 217 +++++++++++++++++++++++++++++++++++++++---------
 io_uring/rsrc.h |  12 +++
 2 files changed, 191 insertions(+), 38 deletions(-)


base-commit: 59b28a6e37e650c0d601ed87875b6217140cda5d
-- 
2.34.1