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.

Signed-off-by: Ming Lei <redacted>
---
 Documentation/block/biovecs.txt | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt
index 25689584e6e0..bb008f7afb05 100644
--- a/Documentation/block/biovecs.txt
+++ b/Documentation/block/biovecs.txt

@@ -117,3 +117,27 @@ 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 bvecs. 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 single-page bvecs. The passed 'struct
+bio_vec' will contain a single-page IO vector during the iteration
+
+	bio_for_each_bvec()