Commit e9538283 authored by Christoph Hellwig's avatar Christoph Hellwig Committed by David Sterba
Browse files

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: default avatarJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: default avatarQu Wenruo <wqu@suse.com>
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent d87e6575

fs/btrfs/extent_io.c

+5 −12
Original line number Diff line number Diff line
@@ -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,
@@ -4361,11 +4360,9 @@ 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)) {

			num_reads++;

		if (!PageUptodate(page))

			all_uptodate = 0;

	}

	}



	if (all_uptodate) {

		set_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);
@@ -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);

		}

	}



	submit_one_bio(&bio_ctrl);