Re: switch loop and target to use ITER_BVEC iov_iter

[Ming Lei <ming.lei@canonical.com>]

On Sun, Jan 18, 2015 at 11:07 PM, Christoph Hellwig <hch@lst.de> wrote:
> This series adds two new helpers to easily read from and write to bio_bvecs,
> and switches the loop driver and target file backend to use it.
>
> Using bio_vecs directly avoids the need to kmap individual elements in
> the callers, which is epecially important in the target driver, and also
> gets rid of the horrible splice code abuse hack in the loop driver.

IMO, from driver or kernel view, submit()/complete() model
is very very common, and is more efficient because unnecessary
context switch and process' creation can be avoided when concurrent
read/write is needed for sake of performance, so I think kernel
aio based API is better. My test result about kernel aio based loop
shows CPU can be saved much with kernel AIO in [2].

Currently the kernel AIO based approach for loop(v2 patches, [1] [2])
only handles direct I/O with ITER_BVEC, but it is quite easy to
cover non-direct I/O by BVEC with supporting current submission.

For encrypt_type loop, it may be covered by kernel aio interface too.

I will try to cover the above two loop cases by kernel based AIO in
v3, then the current ->write() and splice code may be removed once
it is done.

[1] http://marc.info/?l=linux-kernel&m=142116394225654&w=2
[2] http://marc.info/?l=linux-kernel&m=142116397525668&w=2

Thanks,
Ming Lei