Re: [PATCH 1/3] block: fix bio_may_need_split() by using bvec iterator way

[Nitesh Shetty <nj.shetty@samsung.com>]

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On 18/12/25 05:45PM, Ming Lei wrote:
>On Thu, Dec 18, 2025 at 01:37:37AM -0800, Christoph Hellwig wrote:
>> On Thu, Dec 18, 2025 at 05:31:42PM +0800, Ming Lei wrote:
>> > ->bi_vcnt doesn't make sense for cloned bio, which is perfectly fine
>> > passed to bio_may_need_split().
>> >
>> > So fix bio_may_need_split() by not taking ->bi_vcnt directly, instead
>> > checking with help from bio size and bvec->len.
>> >
>> > Meantime retrieving the 1st bvec via __bvec_iter_bvec().
>>
>> That totally misses the point.  The ->bi_vcnt is a fast and lose
>> check to see if we need the fairly expensive iterators to do the
>> real check.
>
>It is just __bvec_iter_bvec(), whatever it should be in cache sooner or
>later.
>
>
Functionality wise overall patch looks fine to me, but as Christoph
stated there is slight performance(IOPS) penalty.
Here is my benchmarking numbers[1], I suspect Jens setup might show
more regression.

Regards,
Nitesh


[1]
===============================
a. two optane nvme device setup:
----------
base case:
----------
sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
-d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nvme0n1 /dev/nvme1n1
submitter=0, tid=206586, file=/dev/nvme0n1, nfiles=1, node=-1
submitter=1, tid=206587, file=/dev/nvme1n1, nfiles=1, node=-1
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=128
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=128
IOPS=6.45M, BW=3.15GiB/s, IOS/call=32/31
IOPS=6.47M, BW=3.16GiB/s, IOS/call=32/32
IOPS=6.47M, BW=3.16GiB/s, IOS/call=32/32
Exiting on timeout
Maximum IOPS=6.47M

----------------
with this patch:
----------------
sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
-d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nvme0n1 /dev/nvme1n1
submitter=0, tid=6352, file=/dev/nvme0n1, nfiles=1, node=-1
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=128
submitter=1, tid=6353, file=/dev/nvme1n1, nfiles=1, node=-1
IOPS=6.30M, BW=3.08GiB/s, IOS/call=32/31
IOPS=6.35M, BW=3.10GiB/s, IOS/call=32/31
IOPS=6.37M, BW=3.11GiB/s, IOS/call=32/32
Exiting on timeout
Maximum IOPS=6.37M

=============================
b. two null-blk device setup:
------------------
null device setup:
------------------
sudo modprobe null_blk queue_mode=2 gb=10 bs=512 nr_devices=2 irqmode=2 \
completion_nsec=1000000 hw_queue_depth=256 memory_backed=0 discard=0 \
use_per_node_hctx=1

----------
base case:
----------
sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
-d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nullb0 /dev/nullb1
submitter=0, tid=6743, file=/dev/nullb0, nfiles=1, node=-1
submitter=1, tid=6744, file=/dev/nullb1, nfiles=1, node=-1
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=128
IOPS=7.89M, BW=3.85GiB/s, IOS/call=32/31
IOPS=7.96M, BW=3.89GiB/s, IOS/call=32/32
IOPS=7.99M, BW=3.90GiB/s, IOS/call=32/32
Exiting on timeout
Maximum IOPS=7.99M

-------------------
with this patchset:
-------------------
sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
-d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nullb0 /dev/nullb1
submitter=0, tid=35633, file=/dev/nullb0, nfiles=1, node=-1
submitter=1, tid=35634, file=/dev/nullb1, nfiles=1, node=-1
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=128
polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
Engine=io_uring, sq_ring=128, cq_ring=128
IOPS=7.79M, BW=3.80GiB/s, IOS/call=32/31
IOPS=7.86M, BW=3.84GiB/s, IOS/call=32/32
IOPS=7.89M, BW=3.85GiB/s, IOS/call=32/32
Exiting on timeout
Maximum IOPS=7.89M

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