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[PATCH v2 20/51] block: introduce multipage/single page bvec helpers

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This patch introduces helpers which are suffixed with _mp
and _sp for the multipage bvec/segment support.

The helpers with _mp suffix are the interfaces for treating
one bvec/segment as real multipage one, for example, .bv_len
is the total length of the multipage segment.

The helpers with _sp suffix are interfaces for supporting
current bvec iterator which is thought as singlepage only
by drivers, fs, dm and etc. These _sp helpers are introduced
to build singlepage bvec in flight, so users of bio/bvec
iterator still can work well and needn't change even though
we store multipage into bvec.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 include/linux/bvec.h | 56 +++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 53 insertions(+), 3 deletions(-)

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 162ca7caf510..f52587e283d4 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -24,6 +24,42 @@
 #include <linux/bug.h>
 
 /*
+ * What is multipage bvecs(segment)?
+ *
+ * - bvec stored in bio->bi_io_vec is always multipage style vector
+ *
+ * - bvec(struct bio_vec) represents one physically contiguous I/O
+ *   buffer, now the buffer may include more than one pages since
+ *   multipage(mp) bvec is supported, and all these pages represented
+ *   by one bvec is physically contiguous. Before mp support, at most
+ *   one page can be included in one bvec, we call it singlepage(sp)
+ *   bvec.
+ *
+ * - .bv_page of th bvec represents the 1st page in the mp segment
+ *
+ * - .bv_offset of the bvec represents 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 bvec with single page
+ *
+ * - bio_for_each_segment_all() will be changed to return singlepage
+ *   bvec too
+ *
+ * - during iterating, iterator variable(struct bvec_iter) is always
+ *   updated in multipage bvec style and that means bvec_iter_advance()
+ *   is kept not changed
+ *
+ * - returned(copied) singlepage bvec is generated in flight by bvec
+ *   helpers from the stored mp bvec
+ *
+ * - In case that some components(such as iov_iter) need to support mp
+ *   segment, we introduce new helpers(suffixed with _mp) for them.
+ */
+
+/*
  * was unsigned short, but we might as well be ready for > 64kB I/O pages
  */
 struct bio_vec {
@@ -49,16 +85,30 @@ struct bvec_iter {
  */
 #define __bvec_iter_bvec(bvec, iter)	(&(bvec)[(iter).bi_idx])
 
-#define bvec_iter_page(bvec, iter)				\
+#define bvec_iter_page_mp(bvec, iter)				\
 	(__bvec_iter_bvec((bvec), (iter))->bv_page)
 
-#define bvec_iter_len(bvec, iter)				\
+#define bvec_iter_len_mp(bvec, iter)				\
 	min((iter).bi_size,					\
 	    __bvec_iter_bvec((bvec), (iter))->bv_len - (iter).bi_bvec_done)
 
-#define bvec_iter_offset(bvec, iter)				\
+#define bvec_iter_offset_mp(bvec, iter)				\
 	(__bvec_iter_bvec((bvec), (iter))->bv_offset + (iter).bi_bvec_done)
 
+/*
+ * <page, offset,length> of singlepage(sp) segment.
+ *
+ * This helpers will be implemented for building sp bvec in flight.
+ */
+#define bvec_iter_offset_sp(bvec, iter)	bvec_iter_offset_mp((bvec), (iter))
+#define bvec_iter_len_sp(bvec, iter)	bvec_iter_len_mp((bvec), (iter))
+#define bvec_iter_page_sp(bvec, iter)	bvec_iter_page_mp((bvec), (iter))
+
+/* current interfaces support sp style at default */
+#define bvec_iter_page(bvec, iter)	bvec_iter_page_sp((bvec), (iter))
+#define bvec_iter_len(bvec, iter)	bvec_iter_len_sp((bvec), (iter))
+#define bvec_iter_offset(bvec, iter)	bvec_iter_offset_sp((bvec), (iter))
+
 #define bvec_iter_bvec(bvec, iter)				\
 ((struct bio_vec) {						\
 	.bv_page	= bvec_iter_page((bvec), (iter)),	\
-- 
2.9.4

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[PATCH v2 00/51] block: support multipage bvec Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:20 +0200
  [PATCH v2 29/51] block: bio: introduce single/multi page version of bio_for_each_segment_all() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:20 +0200
  [PATCH v2 43/51] xfs: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 22/51] block: introduce bio_for_each_segment_mp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 18/51] block: bounce: don't access bio->bi_io_vec in copy_to_high_bio_irq Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 20/51] block: introduce multipage/single page bvec helpers Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 51/51] block: bio: pass segments to bio if bio_add_page() is bypassed Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 32/51] btrfs: use bvec_get_last_page to get bio's last page Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 41/51] fs/iomap: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 14/51] btrfs: avoid to access bvec table directly for a cloned bio Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
    Re: [PATCH v2 14/51] btrfs: avoid to access bvec table directly for  a cloned bio Liu Bo <bo.li.liu@oracle.com> - 2017-06-26 20:10 +0200
  [PATCH v2 46/51] exofs: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 49/51] fs/direct-io: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 39/51] fs/mpage: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 40/51] fs/block: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 37/51] dm-crypt: don't clear bvec->bv_page in crypt_free_buffer_pages() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 47/51] fs: crypto: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 44/51] gfs2: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 45/51] f2fs: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 50/51] block: enable multipage bvecs Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 48/51] fs/btrfs: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 33/51] block: deal with dirtying pages for multipage bvec Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 36/51] md: raid1: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  [PATCH v2 42/51] ext4: convert to bio_for_each_segment_all_sp() Ming Lei <ming.lei@redhat.com> - 2017-06-26 14:30 +0200
  Re: [PATCH v2 00/51] block: support multipage bvec David Sterba <dsterba@suse.cz> - 2017-06-26 18:50 +0200
  Re: [PATCH v2 00/51] block: support multipage bvec Jens Axboe <axboe@kernel.dk> - 2017-06-26 18:50 +0200

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