btrfs: simplify main loop in submit_extent_page

bio_add_page always adds either the entire range passed to it or nothing.
Based on that btrfs_bio_add_page can only return a length smaller than
the passed in one when hitting the ordered extent limit, which can only
happen for writes.  Given that compressed writes never even use this code
path, this means that all the special cases for compressed extent offset
handling are dead code.

Reflow submit_extent_page to take advantage of this by inlining
btrfs_bio_add_page and handling the ordered extent limit by decrementing
it for each added range and thus significantly simplifying the loop.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -898,47 +898,6 @@ static bool btrfs_bio_is_contig(struct btrfs_bio_ctrl *bio_ctrl,
 		page_offset(page) + pg_offset;
 }
 
-/*
- * Attempt to add a page to bio.
- *
- * @bio_ctrl:       record both the bio, and its bio_flags
- * @page:	    page to add to the bio
- * @disk_bytenr:    offset of the new bio or to check whether we are adding
- *                  a contiguous page to the previous one
- * @size:	    portion of page that we want to write
- * @pg_offset:	    starting offset in the page
- *
- * Attempt to add a page to bio considering stripe alignment etc.
- *
- * Return >= 0 for the number of bytes added to the bio.
- * Can return 0 if the current bio is already at stripe/zone boundary.
- * Return <0 for error.
- */
-static int btrfs_bio_add_page(struct btrfs_bio_ctrl *bio_ctrl,
-			      struct page *page,
-			      u64 disk_bytenr, unsigned int size,
-			      unsigned int pg_offset)
-{
-	struct bio *bio = bio_ctrl->bio;
-	u32 bio_size = bio->bi_iter.bi_size;
-	u32 real_size;
-
-	ASSERT(bio);
-	/* The limit should be calculated when bio_ctrl->bio is allocated */
-	ASSERT(bio_ctrl->len_to_oe_boundary);
-
-	real_size = min(bio_ctrl->len_to_oe_boundary - bio_size, size);
-
-	/*
-	 * If real_size is 0, never call bio_add_*_page(), as even size is 0,
-	 * bio will still execute its endio function on the page!
-	 */
-	if (real_size == 0)
-		return 0;
-
-	return bio_add_page(bio, page, real_size, pg_offset);
-}
-
 static void calc_bio_boundaries(struct btrfs_bio_ctrl *bio_ctrl,
 				struct btrfs_inode *inode, u64 file_offset)
 {
@@ -966,21 +925,14 @@ static void calc_bio_boundaries(struct btrfs_bio_ctrl *bio_ctrl,
 
 static void alloc_new_bio(struct btrfs_inode *inode,
 			  struct btrfs_bio_ctrl *bio_ctrl,
-			  u64 disk_bytenr, u32 offset, u64 file_offset)
+			  u64 disk_bytenr, u64 file_offset)
 {
 	struct btrfs_fs_info *fs_info = inode->root->fs_info;
 	struct bio *bio;
 
 	bio = btrfs_bio_alloc(BIO_MAX_VECS, bio_ctrl->opf, inode,
 			      bio_ctrl->end_io_func, NULL);
-	/*
-	 * For compressed page range, its disk_bytenr is always @disk_bytenr
-	 * passed in, no matter if we have added any range into previous bio.
-	 */
-	if (bio_ctrl->compress_type != BTRFS_COMPRESS_NONE)
 	bio->bi_iter.bi_sector = disk_bytenr >> SECTOR_SHIFT;
-	else
-		bio->bi_iter.bi_sector = (disk_bytenr + offset) >> SECTOR_SHIFT;
 	btrfs_bio(bio)->file_offset = file_offset;
 	bio_ctrl->bio = bio;
 	calc_bio_boundaries(bio_ctrl, inode, file_offset);
@@ -1014,56 +966,48 @@ static void submit_extent_page(struct btrfs_bio_ctrl *bio_ctrl,
 			       size_t size, unsigned long pg_offset)
 {
 	struct btrfs_inode *inode = BTRFS_I(page->mapping->host);
-	unsigned int cur = pg_offset;
-
-	ASSERT(bio_ctrl);
-
-	ASSERT(pg_offset < PAGE_SIZE && size <= PAGE_SIZE &&
-	       pg_offset + size <= PAGE_SIZE);
 
+	ASSERT(pg_offset + size <= PAGE_SIZE);
 	ASSERT(bio_ctrl->end_io_func);
 
 	if (bio_ctrl->bio &&
 	    !btrfs_bio_is_contig(bio_ctrl, page, disk_bytenr, pg_offset))
 		submit_one_bio(bio_ctrl);
 
-	while (cur < pg_offset + size) {
-		u32 offset = cur - pg_offset;
-		int added;
+	do {
+		u32 len = size;
 
 		/* Allocate new bio if needed */
 		if (!bio_ctrl->bio) {
 			alloc_new_bio(inode, bio_ctrl, disk_bytenr,
-				      offset, page_offset(page) + cur);
+				      page_offset(page) + pg_offset);
 		}
-		/*
-		 * We must go through btrfs_bio_add_page() to ensure each
-		 * page range won't cross various boundaries.
-		 */
-		if (bio_ctrl->compress_type != BTRFS_COMPRESS_NONE)
-			added = btrfs_bio_add_page(bio_ctrl, page, disk_bytenr,
-					size - offset, pg_offset + offset);
-		else
-			added = btrfs_bio_add_page(bio_ctrl, page,
-					disk_bytenr + offset, size - offset,
-					pg_offset + offset);
-
-		/* Metadata page range should never be split */
-		if (!is_data_inode(&inode->vfs_inode))
-			ASSERT(added == 0 || added == size - offset);
-
-		/* At least we added some page, update the account */
-		if (bio_ctrl->wbc && added)
-			wbc_account_cgroup_owner(bio_ctrl->wbc, page, added);
-
-		/* We have reached boundary, submit right now */
-		if (added < size - offset) {
-			/* The bio should contain some page(s) */
-			ASSERT(bio_ctrl->bio->bi_iter.bi_size);
-			submit_one_bio(bio_ctrl);
+
+		/* Cap to the current ordered extent boundary if there is one. */
+		if (len > bio_ctrl->len_to_oe_boundary) {
+			ASSERT(bio_ctrl->compress_type == BTRFS_COMPRESS_NONE);
+			ASSERT(is_data_inode(&inode->vfs_inode));
+			len = bio_ctrl->len_to_oe_boundary;
 		}
-		cur += added;
+
+		if (bio_add_page(bio_ctrl->bio, page, len, pg_offset) != len) {
+			/* bio full: move on to a new one */
+			submit_one_bio(bio_ctrl);
+			continue;
 		}
+
+		if (bio_ctrl->wbc)
+			wbc_account_cgroup_owner(bio_ctrl->wbc, page, len);
+
+		size -= len;
+		pg_offset += len;
+		disk_bytenr += len;
+		bio_ctrl->len_to_oe_boundary -= len;
+
+		/* Ordered extent boundary: move on to a new bio. */
+		if (bio_ctrl->len_to_oe_boundary == 0)
+			submit_one_bio(bio_ctrl);
+	} while (size);
 }
 
 static int attach_extent_buffer_page(struct extent_buffer *eb,