block: add a lower-level bio_add_page interface

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>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

1 files changed, 63 insertions, 33 deletions

diff --git a/block/bio.c b/block/bio.c
index 53e0f0a..fdf635d 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;
 
 	/*
@@ -821,52 +821,82 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
 EXPORT_SYMBOL(bio_add_pc_page);
 
 /**
- *	bio_add_page	-	attempt to add page to bio
- *	@bio: destination bio
- *	@page: page to add
- *	@len: vec entry length
- *	@offset: vec entry offset
+ * __bio_try_merge_page - try appending data to an existing bvec.
+ * @bio: destination bio
+ * @page: page to add
+ * @len: length of the data to add
+ * @off: offset of the data in @page
  *
- *	Attempt to add a page to the bio_vec maplist. This will only fail
- *	if either bio->bi_vcnt == bio->bi_max_vecs or it's a cloned bio.
+ * Try to add the data at @page + @off to the last bvec of @bio.  This is a
+ * a useful optimisation for file systems with a block size smaller than the
+ * page size.
+ *
+ * Return %true on success or %false on failure.
  */
-int bio_add_page(struct bio *bio, struct page *page,
-		 unsigned int len, unsigned int offset)
+bool __bio_try_merge_page(struct bio *bio, struct page *page,
+		unsigned int len, unsigned int off)
 {
-	struct bio_vec *bv;
-
-	/*
-	 * cloned bio must not modify vec list
-	 */
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
-		return 0;
+		return false;
 
-	/*
-	 * 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];
+		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
 
-		if (page == bv->bv_page &&
-		    offset == bv->bv_offset + bv->bv_len) {
+		if (page == bv->bv_page && off == bv->bv_offset + bv->bv_len) {
 			bv->bv_len += len;
-			goto done;
+			bio->bi_iter.bi_size += len;
+			return true;
 		}
 	}
+	return false;
+}
+EXPORT_SYMBOL_GPL(__bio_try_merge_page);
 
-	if (bio->bi_vcnt >= bio->bi_max_vecs)
-		return 0;
+/**
+ * __bio_add_page - add page to a bio in a new segment
+ * @bio: destination bio
+ * @page: page to add
+ * @len: length of the data to add
+ * @off: offset of the data in @page
+ *
+ * Add the data at @page + @off 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];
 
-	bv		= &bio->bi_io_vec[bio->bi_vcnt];
-	bv->bv_page	= page;
-	bv->bv_len	= len;
-	bv->bv_offset	= offset;
+	WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED));
+	WARN_ON_ONCE(bio_full(bio));
+
+	bv->bv_page = page;
+	bv->bv_offset = off;
+	bv->bv_len = len;
 
-	bio->bi_vcnt++;
-done:
 	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
+ *	@page: page to add
+ *	@len: vec entry length
+ *	@offset: vec entry offset
+ *
+ *	Attempt to add a page to the bio_vec maplist. This will only fail
+ *	if either bio->bi_vcnt == bio->bi_max_vecs or it's a cloned bio.
+ */
+int bio_add_page(struct bio *bio, struct page *page,
+		 unsigned int len, unsigned int offset)
+{
+	if (!__bio_try_merge_page(bio, page, len, offset)) {
+		if (bio_full(bio))
+			return 0;
+		__bio_add_page(bio, page, len, offset);
+	}
 	return len;
 }
 EXPORT_SYMBOL(bio_add_page);