Author: Chris Mason[off-list ref]
Date:   Wed Feb 22 12:36:24 2012 -0500

    Btrfs: clear the extent uptodate bits during parent transid failures

    If btrfs reads a block and finds a parent transid mismatch, it clears
    the uptodate flags on the extent buffer, and the pages inside it.  But
    we only clear the uptodate bits in the state tree if the block straddles
    more than one page.

    This is from an old optimization from to reduce contention on the extent
    state tree.  But it is buggy because the code that retries a read from
    a different copy of the block is going to find the uptodate state bits
    set and skip the IO.

    The end result of the bug is that we'll never actually read the good
    copy (if there is one).

    The fix here is to always clear the uptodate state bits, which is safe
    because this code is only called when the parent transid fails.

    Signed-off-by: Chris Mason[off-list ref]

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 1e8d5e5..a4dc892 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c

@@ -3852,10 +3852,9 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree,
 	num_pages = num_extent_pages(eb->start, eb->len);
 	clear_bit(EXTENT_BUFFER_UPTODATE,&eb->bflags);

-	if (eb_straddles_pages(eb)) {
-		clear_extent_uptodate(tree, eb->start, eb->start + eb->len - 1,
-				      cached_state, GFP_NOFS);
-	}
+	clear_extent_uptodate(tree, eb->start, eb->start + eb->len - 1,
+			      cached_state, GFP_NOFS);
+
 	for (i = 0; i<  num_pages; i++) {
 		page = extent_buffer_page(eb, i);
 		if (page)
--

--

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