btrfs: Always try all copies when reading extent buffers

When a metadata read is served the endio routine btree_readpage_end_io_hook
is called which eventually runs the tree-checker. If tree-checker fails
to validate the read eb then it sets EXTENT_BUFFER_CORRUPT flag. This
leads to btree_read_extent_buffer_pages wrongly assuming that all
available copies of this extent buffer are wrong and failing prematurely.
Fix this modify btree_read_extent_buffer_pages to read all copies of
the data.

This failure was exhibitted in xfstests btrfs/124 which would
spuriously fail its balance operations. The reason was that when balance
was run following re-introduction of the missing raid1 disk
__btrfs_map_block would map the read request to stripe 0, which
corresponded to devid 2 (the disk which is being removed in the test):

    item 2 key (FIRST_CHUNK_TREE CHUNK_ITEM 3553624064) itemoff 15975 itemsize 112
	length 1073741824 owner 2 stripe_len 65536 type DATA|RAID1
	io_align 65536 io_width 65536 sector_size 4096
	num_stripes 2 sub_stripes 1
		stripe 0 devid 2 offset 2156920832
		dev_uuid 8466c350-ed0c-4c3b-b17d-6379b445d5c8
		stripe 1 devid 1 offset 3553624064
		dev_uuid 1265d8db-5596-477e-af03-df08eb38d2ca

This caused read requests for a checksum item that to be routed to the
stale disk which triggered the aforementioned logic involving
EXTENT_BUFFER_CORRUPT flag. This then triggered cascading failures of
the balance operation.

Fixes: a826d6dc ("Btrfs: check items for correctness as we search")
CC: stable@vger.kernel.org # 4.4+
Suggested-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -477,9 +477,9 @@ static int btree_read_extent_buffer_pages(struct btrfs_fs_info *fs_info,
 	int mirror_num = 0;
 	int failed_mirror = 0;
 
-	clear_bit(EXTENT_BUFFER_CORRUPT, &eb->bflags);
 	io_tree = &BTRFS_I(fs_info->btree_inode)->io_tree;
 	while (1) {
+		clear_bit(EXTENT_BUFFER_CORRUPT, &eb->bflags);
 		ret = read_extent_buffer_pages(io_tree, eb, WAIT_COMPLETE,
 					       mirror_num);
 		if (!ret) {
@@ -493,15 +493,6 @@ static int btree_read_extent_buffer_pages(struct btrfs_fs_info *fs_info,
 				break;
 		}
 
-		/*
-		 * This buffer's crc is fine, but its contents are corrupted, so
-		 * there is no reason to read the other copies, they won't be
-		 * any less wrong.
-		 */
-		if (test_bit(EXTENT_BUFFER_CORRUPT, &eb->bflags) ||
-		    ret == -EUCLEAN)
-			break;
-
 		num_copies = btrfs_num_copies(fs_info,
 					      eb->start, eb->len);
 		if (num_copies == 1)