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Date:   Fri, 15 Feb 2019 19:13:22 +0800
From:   Ming Lei <>
To:     Jens Axboe <>
Cc:,,, Theodore Ts'o <>,
        Omar Sandoval <>,
        Sagi Grimberg <>,
        Dave Chinner <>,
        Kent Overstreet <>,
        Mike Snitzer <>,,
        Alexander Viro <>,,,
        David Sterba <>,,
        "Darrick J . Wong" <>,, Gao Xiang <>,
        Christoph Hellwig <>,,
        Coly Li <>,,
        Boaz Harrosh <>,
        Bob Peterson <>,,
        Ming Lei <>
Subject: [PATCH V15 16/18] 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 <>
Reviewed-by: Omar Sandoval <>
Signed-off-by: Ming Lei <>
 Documentation/block/biovecs.txt | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt
index 25689584e6e0..ce6eccaf5df7 100644
--- a/Documentation/block/biovecs.txt
+++ b/Documentation/block/biovecs.txt
@@ -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
+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
+	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()

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