Patch "btrfs: always read the entire extent_buffer" has been added to the 6.4-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    btrfs: always read the entire extent_buffer

to the 6.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     btrfs-always-read-the-entire-extent_buffer.patch
and it can be found in the queue-6.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 7fa6fa50bcf63d8f4c6a381d7fb7a2ffc0521572
Author: Christoph Hellwig <hch@xxxxxx>
Date:   Wed May 3 17:24:25 2023 +0200

    btrfs: always read the entire extent_buffer
    
    [ Upstream commit e95382834cf885b478dbe14a66451b863eb35c94 ]
    
    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@xxxxxxx>
    Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
    Signed-off-by: Christoph Hellwig <hch@xxxxxx>
    Reviewed-by: David Sterba <dsterba@xxxxxxxx>
    Signed-off-by: David Sterba <dsterba@xxxxxxxx>
    Stable-dep-of: 7027f87108ce ("btrfs: don't treat zoned writeback as being from an async helper thread")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index a1adadd5d25dd..d1a635f237688 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4314,7 +4314,6 @@ 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,
@@ -4360,10 +4359,8 @@ 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) {
@@ -4373,7 +4370,7 @@ 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.
@@ -4383,13 +4380,9 @@ 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);