[PATCH V15 04/18] block: introduce multi-page bvec helpers

From: Ming Lei
Date: Fri Feb 15 2019 - 06:14:54 EST

This patch introduces helpers of 'mp_bvec_iter_*' for multi-page bvec

The introduced helpers treate one bvec as real multi-page segment,
which may include more than one pages.

The existed helpers of bvec_iter_* are interfaces for supporting current
bvec iterator which is thought as single-page by drivers, fs, dm and
etc. These introduced helpers will build single-page bvec in flight, so
this way won't break current bio/bvec users, which needn't any change.

Follows some multi-page bvec background:

- bvecs stored in bio->bi_io_vec is always multi-page style

- bvec(struct bio_vec) represents one physically contiguous I/O
buffer, now the buffer may include more than one page after
multi-page bvec is supported, and all these pages represented
by one bvec is physically contiguous. Before multi-page bvec
support, at most one page is included in one bvec, we call it
single-page bvec.

- .bv_page of the bvec points to the 1st page in the multi-page bvec

- .bv_offset of the bvec is the offset of the buffer in the bvec

The effect on the current drivers/filesystem/dm/bcache/...:

- almost everyone supposes that one bvec only includes one single
page, so we keep the sp interface not changed, for example,
bio_for_each_segment() still returns single-page bvec

- bio_for_each_segment_all() will return single-page bvec too

- during iterating, iterator variable(struct bvec_iter) is always
updated in multi-page bvec style, and bvec_iter_advance() is kept
not changed

- returned(copied) single-page bvec is built in flight by bvec
helpers from the stored multi-page bvec

Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Omar Sandoval <osandov@xxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
include/linux/bvec.h | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index ba0ae40e77c9..0ae729b1c9fe 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -23,6 +23,7 @@
#include <linux/kernel.h>
#include <linux/bug.h>
#include <linux/errno.h>
+#include <linux/mm.h>

* was unsigned short, but we might as well be ready for > 64kB I/O pages
@@ -50,16 +51,39 @@ struct bvec_iter {
#define __bvec_iter_bvec(bvec, iter) (&(bvec)[(iter).bi_idx])

-#define bvec_iter_page(bvec, iter) \
+/* multi-page (mp_bvec) helpers */
+#define mp_bvec_iter_page(bvec, iter) \
(__bvec_iter_bvec((bvec), (iter))->bv_page)

-#define bvec_iter_len(bvec, iter) \
+#define mp_bvec_iter_len(bvec, iter) \
min((iter).bi_size, \
__bvec_iter_bvec((bvec), (iter))->bv_len - (iter).bi_bvec_done)

-#define bvec_iter_offset(bvec, iter) \
+#define mp_bvec_iter_offset(bvec, iter) \
(__bvec_iter_bvec((bvec), (iter))->bv_offset + (iter).bi_bvec_done)

+#define mp_bvec_iter_page_idx(bvec, iter) \
+ (mp_bvec_iter_offset((bvec), (iter)) / PAGE_SIZE)
+#define mp_bvec_iter_bvec(bvec, iter) \
+((struct bio_vec) { \
+ .bv_page = mp_bvec_iter_page((bvec), (iter)), \
+ .bv_len = mp_bvec_iter_len((bvec), (iter)), \
+ .bv_offset = mp_bvec_iter_offset((bvec), (iter)), \
+/* For building single-page bvec in flight */
+ #define bvec_iter_offset(bvec, iter) \
+ (mp_bvec_iter_offset((bvec), (iter)) % PAGE_SIZE)
+#define bvec_iter_len(bvec, iter) \
+ min_t(unsigned, mp_bvec_iter_len((bvec), (iter)), \
+ PAGE_SIZE - bvec_iter_offset((bvec), (iter)))
+#define bvec_iter_page(bvec, iter) \
+ nth_page(mp_bvec_iter_page((bvec), (iter)), \
+ mp_bvec_iter_page_idx((bvec), (iter)))
#define bvec_iter_bvec(bvec, iter) \
((struct bio_vec) { \
.bv_page = bvec_iter_page((bvec), (iter)), \