linux-kernel - [PATCH V9 16/19] block: document usage of bio iterator helpers

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Date:   Tue, 13 Nov 2018 23:15:02 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>, Christoph Hellwig <hch@....de>
Subject: [PATCH V9 16/19] 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.

Cc: Christoph Hellwig <hch@....de>
Signed-off-by: Ming Lei <ming.lei@...hat.com>
---
 Documentation/block/biovecs.txt | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt
index 25689584e6e0..bfafb70d0d9e 100644
--- a/Documentation/block/biovecs.txt
+++ b/Documentation/block/biovecs.txt
@@ -117,3 +117,29 @@ 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, and usually they are used by filesystem code, and driver
+shouldn't use them because bio may have been split before they got to 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 bvec, and the local
+variable of 'struct bio_vec' or the reference records single-page IO
+vector during the itearation:
+
+	bio_for_each_segment()
+	bio_for_each_segment_all()
+
+* The following helper iterates over multi-page bvec, and each bvec may
+include multiple physically contiguous pages, and the local variable of
+'struct bio_vec' or the reference records multi-page IO vector during the
+itearation:
+
+	bio_for_each_bvec()
-- 
2.9.5