linux-kernel - [RFC PATCH 17/17] Documentation: update notes in biovecs about arbitrarily sized bios

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Message-Id: <a116a7da8f881f69dfcc2e40bda91533f1bb3c57.1419241597.git.dongsu.park@profitbricks.com>
Date:	Mon, 22 Dec 2014 12:48:44 +0100
From:	Dongsu Park <dongsu.park@...fitbricks.com>
To:	linux-kernel@...r.kernel.org
Cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	Ming Lin <mlin@...ggr.net>,
	Dongsu Park <dongsu.park@...fitbricks.com>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: [RFC PATCH 17/17] Documentation: update notes in biovecs about arbitrarily sized bios

Update block/biovecs.txt so that it includes a note on what kind of
effects arbitrarily sized bios would bring to the block layer.
Also fix a trivial typo, bio_iter_iovec.

Signed-off-by: Dongsu Park <dongsu.park@...fitbricks.com>
Cc: Kent Overstreet <kmo@...erainc.com>
Cc: Jonathan Corbet <corbet@....net>
Cc: linux-doc@...r.kernel.org
---
 Documentation/block/biovecs.txt | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/Documentation/block/biovecs.txt b/Documentation/block/biovecs.txt
index 74a32ad..339045d 100644
--- a/Documentation/block/biovecs.txt
+++ b/Documentation/block/biovecs.txt
@@ -24,7 +24,7 @@ particular, presenting the illusion of partially completed biovecs so that
 normal code doesn't have to deal with bi_bvec_done.
 
  * Driver code should no longer refer to biovecs directly; we now have
-   bio_iovec() and bio_iovec_iter() macros that return literal struct biovecs,
+   bio_iovec() and bio_iter_iovec() macros that return literal struct biovecs,
    constructed from the raw biovecs but taking into account bi_bvec_done and
    bi_size.
 
@@ -109,3 +109,18 @@ Other implications:
    over all the biovecs in the new bio - which is silly as it's not needed.
 
    So, don't use bi_vcnt anymore.
+
+ * As of 3.18, block layer is written based on merging biovecs. Its goal is
+   to avoid having to split bios; upper layer code such as bio_add_page()
+   checks what the underlying device can handle, and tries to always create
+   bios that don't need to be split. However, this approach has been actually
+   cumbersome and error-prone. It eventually breaks down with stacked devices
+   and devices with dynamic limits, which then adds a lot of complexity.
+
+   So its new interface allows the block layer to split bios as needed, so we
+   could eliminate a lot of complexity elsewhere - particularly in stacked
+   drivers. Code that creates bios can then create whatever size bios are
+   convenient, and more importantly stacked drivers don't have to deal with
+   both their own bio size limitations and the limitations of the underlying
+   devices. Thus there's no need to define ->merge_bvec_fn() callbacks for
+   individual block drivers.
-- 
2.1.0

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