[PATCH 01/33] block: add a lower-level bio_add_page interface

[Christoph Hellwig <hch@lst.de>]

For the upcoming removal of buffer heads in XFS we need to keep track of
the number of outstanding writeback requests per page.  For this we need
to know if bio_add_page merged a region with the previous bvec or not.
Instead of adding additional arguments this refactors bio_add_page to
be implemented using three lower level helpers which users like XFS can
use directly if they care about the merge decisions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/bio.c         | 87 ++++++++++++++++++++++++++++++---------------
 include/linux/bio.h |  9 +++++
 2 files changed, 67 insertions(+), 29 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 53e0f0a1ed94..6ceba6adbf42 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -773,7 +773,7 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
 			return 0;
 	}
 
-	if (bio->bi_vcnt >= bio->bi_max_vecs)
+	if (bio_full(bio))
 		return 0;
 
 	/*
@@ -820,6 +820,59 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
 }
 EXPORT_SYMBOL(bio_add_pc_page);
 
+/**
+ * __bio_try_merge_page - try adding data to an existing bvec
+ * @bio: destination bio
+ * @page: page to add
+ * @len: length of the range to add
+ * @off: offset into @page
+ *
+ * Try adding the data described at @page + @offset to the last bvec of @bio.
+ * Return %true on success or %false on failure.  This can happen frequently
+ * for file systems with a block size smaller than the page size.
+ */
+bool __bio_try_merge_page(struct bio *bio, struct page *page,
+		unsigned int len, unsigned int off)
+{
+	if (bio->bi_vcnt > 0) {
+		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
+
+		if (page == bv->bv_page && off == bv->bv_offset + bv->bv_len) {
+			bv->bv_len += len;
+			bio->bi_iter.bi_size += len;
+			return true;
+		}
+	}
+	return false;
+}
+EXPORT_SYMBOL_GPL(__bio_try_merge_page);
+
+/**
+ * __bio_add_page - add page to a bio in a new segment
+ * @bio: destination bio
+ * @page: page to add
+ * @len: length of the range to add
+ * @off: offset into @page
+ *
+ * Add the data at @page + @offset to @bio as a new bvec.  The caller must
+ * ensure that @bio has space for another bvec.
+ */
+void __bio_add_page(struct bio *bio, struct page *page,
+		unsigned int len, unsigned int off)
+{
+	struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt];
+
+	WARN_ON_ONCE(bio_full(bio));
+
+	bv->bv_page = page;
+	bv->bv_offset = off;
+	bv->bv_len = len;
+
+	bio->bi_iter.bi_size += len;
+	bio->bi_vcnt++;
+}
+EXPORT_SYMBOL_GPL(__bio_add_page);
+
 /**
  *	bio_add_page	-	attempt to add page to bio
  *	@bio: destination bio
@@ -833,40 +886,16 @@ EXPORT_SYMBOL(bio_add_pc_page);
 int bio_add_page(struct bio *bio, struct page *page,
 		 unsigned int len, unsigned int offset)
 {
-	struct bio_vec *bv;
-
 	/*
 	 * cloned bio must not modify vec list
 	 */
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
 		return 0;
-
-	/*
-	 * For filesystems with a blocksize smaller than the pagesize
-	 * we will often be called with the same page as last time and
-	 * a consecutive offset.  Optimize this special case.
-	 */
-	if (bio->bi_vcnt > 0) {
-		bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
-
-		if (page == bv->bv_page &&
-		    offset == bv->bv_offset + bv->bv_len) {
-			bv->bv_len += len;
-			goto done;
-		}
+	if (!__bio_try_merge_page(bio, page, len, offset)) {
+		if (bio_full(bio))
+			return 0;
+		__bio_add_page(bio, page, len, offset);
 	}
-
-	if (bio->bi_vcnt >= bio->bi_max_vecs)
-		return 0;
-
-	bv		= &bio->bi_io_vec[bio->bi_vcnt];
-	bv->bv_page	= page;
-	bv->bv_len	= len;
-	bv->bv_offset	= offset;
-
-	bio->bi_vcnt++;
-done:
-	bio->bi_iter.bi_size += len;
 	return len;
 }
 EXPORT_SYMBOL(bio_add_page);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index ce547a25e8ae..3e73c8bc25ea 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -123,6 +123,11 @@ static inline void *bio_data(struct bio *bio)
 	return NULL;
 }
 
+static inline bool bio_full(struct bio *bio)
+{
+	return bio->bi_vcnt >= bio->bi_max_vecs;
+}
+
 /*
  * will die
  */
@@ -470,6 +475,10 @@ void bio_chain(struct bio *, struct bio *);
 extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
 extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
 			   unsigned int, unsigned int);
+bool __bio_try_merge_page(struct bio *bio, struct page *page,
+		unsigned int len, unsigned int off);
+void __bio_add_page(struct bio *bio, struct page *page,
+		unsigned int len, unsigned int off);
 int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter);
 struct rq_map_data;
 extern struct bio *bio_map_user_iov(struct request_queue *,
-- 
2.17.0