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block: document usage of bio iterator helpers 

Now multi-page bvec is supported, some helpers may return page by
page, meantime some may return segment by segment, this patch
documents the usage.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Ming Lei 2019-02-15 19:13:22 +08:00 committed by Jens Axboe
parent 6861428921
commit ac4fa1d107
1 changed files with 25 additions and 0 deletions
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25
Documentation/block/biovecs.txt
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@ -117,3 +117,28 @@ Other implications:
size limitations and the limitations of the underlying devices. Thus
there's no need to define ->merge_bvec_fn() callbacks for individual block
drivers.
Usage of helpers:
=================
* The following helpers whose names have the suffix of "_all" can only be used
on non-BIO_CLONED bio. They are usually used by filesystem code. Drivers
shouldn't use them because the bio may have been split before it reached the
driver.
bio_for_each_segment_all()
bio_first_bvec_all()
bio_first_page_all()
bio_last_bvec_all()
* The following helpers iterate over single-page segment. The passed 'struct
bio_vec' will contain a single-page IO vector during the iteration
bio_for_each_segment()
bio_for_each_segment_all()
* The following helpers iterate over multi-page bvec. The passed 'struct
bio_vec' will contain a multi-page IO vector during the iteration
bio_for_each_bvec()
rq_for_each_bvec()