1 files changed, 56 insertions, 5 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 1b6cd937f214..486b1da2fc5c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -48,6 +48,7 @@
 #include "qgroup.h"
 #include "delalloc-space.h"
 #include "block-group.h"
+#include "space-info.h"
 
 struct btrfs_iget_args {
 	u64 ino;
@@ -1354,6 +1355,56 @@ static noinline int csum_exist_in_range(struct btrfs_fs_info *fs_info,
 	return 1;
 }
 
+static int fallback_to_cow(struct inode *inode, struct page *locked_page,
+			   const u64 start, const u64 end,
+			   int *page_started, unsigned long *nr_written)
+{
+	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
+	u64 range_start = start;
+	u64 count;
+
+	/*
+	 * If EXTENT_NORESERVE is set it means that when the buffered write was
+	 * made we had not enough available data space and therefore we did not
+	 * reserve data space for it, since we though we could do NOCOW for the
+	 * respective file range (either there is prealloc extent or the inode
+	 * has the NOCOW bit set).
+	 *
+	 * However when we need to fallback to COW mode (because for example the
+	 * block group for the corresponding extent was turned to RO mode by a
+	 * scrub or relocation) we need to do the following:
+	 *
+	 * 1) We increment the bytes_may_use counter of the data space info.
+	 *    If COW succeeds, it allocates a new data extent and after doing
+	 *    that it decrements the space info's bytes_may_use counter and
+	 *    increments its bytes_reserved counter by the same amount (we do
+	 *    this at btrfs_add_reserved_bytes()). So we need to increment the
+	 *    bytes_may_use counter to compensate (when space is reserved at
+	 *    buffered write time, the bytes_may_use counter is incremented);
+	 *
+	 * 2) We clear the EXTENT_NORESERVE bit from the range. We do this so
+	 *    that if the COW path fails for any reason, it decrements (through
+	 *    extent_clear_unlock_delalloc()) the bytes_may_use counter of the
+	 *    data space info, which we incremented in the step above.
+	 */
+	count = count_range_bits(io_tree, &range_start, end, end + 1 - start,
+				 EXTENT_NORESERVE, 0);
+	if (count > 0) {
+		struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
+		struct btrfs_space_info *sinfo = fs_info->data_sinfo;
+
+		spin_lock(&sinfo->lock);
+		btrfs_space_info_update_bytes_may_use(fs_info, sinfo, count);
+		spin_unlock(&sinfo->lock);
+
+		clear_extent_bit(io_tree, start, end, EXTENT_NORESERVE, 0, 0,
+				 NULL);
+	}
+
+	return cow_file_range(inode, locked_page, start, end, page_started,
+			      nr_written, 1);
+}
+
 /*
  * when nowcow writeback call back.  This checks for snapshots or COW copies
  * of the extents that exist in the file, and COWs the file as required.
@@ -1601,9 +1652,9 @@ out_check:
 		 * NOCOW, following one which needs to be COW'ed
 		 */
 		if (cow_start != (u64)-1) {
-			ret = cow_file_range(inode, locked_page,
-					     cow_start, found_key.offset - 1,
-					     page_started, nr_written, 1);
+			ret = fallback_to_cow(inode, locked_page, cow_start,
+					      found_key.offset - 1,
+					      page_started, nr_written);
 			if (ret) {
 				if (nocow)
 					btrfs_dec_nocow_writers(fs_info,
@@ -1692,8 +1743,8 @@ out_check:
 
 	if (cow_start != (u64)-1) {
 		cur_offset = end;
-		ret = cow_file_range(inode, locked_page, cow_start, end,
-				     page_started, nr_written, 1);
+		ret = fallback_to_cow(inode, locked_page, cow_start, end,
+				      page_started, nr_written);
 		if (ret)
 			goto error;
 	}