btrfs: always read the entire extent_buffer

Currently read_extent_buffer_pages skips pages that are already uptodate
when reading in an extent_buffer.  While this reduces the amount of data
read, it increases the number of I/O operations as we now need to do
multiple I/Os when reading an extent buffer with one or more uptodate
pages in the middle of it.  On any modern storage device, be that hard
drives or SSDs this actually decreases I/O performance.  Fortunately
this case is pretty rare as the pages are always initially read together
and then aged the same way.  Besides simplifying the code a bit as-is
this will allow for major simplifications to the I/O completion handler
later on.

Note that the case where all pages are uptodate is still handled by an
optimized fast path that does not read any data from disk.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.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

@@ -4315,7 +4315,6 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
 	int locked_pages = 0;
 	int all_uptodate = 1;
 	int num_pages;
-	unsigned long num_reads = 0;
 	struct btrfs_bio_ctrl bio_ctrl = {
 		.opf = REQ_OP_READ,
 		.mirror_num = mirror_num,
@@ -4361,10 +4360,8 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
 	 */
 	for (i = 0; i < num_pages; i++) {
 		page = eb->pages[i];
-		if (!PageUptodate(page)) {
-			num_reads++;
+		if (!PageUptodate(page))
 			all_uptodate = 0;
-		}
 	}
 
 	if (all_uptodate) {
@@ -4374,7 +4371,7 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
 
 	clear_bit(EXTENT_BUFFER_READ_ERR, &eb->bflags);
 	eb->read_mirror = 0;
-	atomic_set(&eb->io_pages, num_reads);
+	atomic_set(&eb->io_pages, num_pages);
 	/*
 	 * It is possible for release_folio to clear the TREE_REF bit before we
 	 * set io_pages. See check_buffer_tree_ref for a more detailed comment.
@@ -4384,13 +4381,9 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num,
 	for (i = 0; i < num_pages; i++) {
 		page = eb->pages[i];
 
-		if (!PageUptodate(page)) {
-			ClearPageError(page);
-			submit_extent_page(&bio_ctrl, page_offset(page), page,
-					   PAGE_SIZE, 0);
-		} else {
-			unlock_page(page);
-		}
+		ClearPageError(page);
+		submit_extent_page(&bio_ctrl, page_offset(page), page,
+				   PAGE_SIZE, 0);
 	}
 
 	submit_one_bio(&bio_ctrl);