btrfs: make btrfs_check_nocow_lock() check more than one extent

Currently btrfs_check_nocow_lock() stops at the first extent it finds and
that extent may be smaller than the target range we want to NOCOW into.
But we can have multiple consecutive extents which we can NOCOW into, so
by stopping at the first one we find we just make the caller do more work
by splitting the write into multiple ones, or in the case of mmap writes
with large folios we fail with -ENOSPC in case the folio's range is
covered by more than one extent (the fallback to NOCOW for mmap writes in
case there's no available data space to reserve/allocate was recently
added by the patch "btrfs: fix -ENOSPC mmap write failure on NOCOW
files/extents").

Improve on this by checking for multiple consecutive extents.

Reviewed-by: Boris Burkov <[e-mail]>
Signed-off-by: Filipe Manana <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/file.c

···
 	struct btrfs_root *root = inode->root;
 	struct extent_state *cached_state = NULL;
 	u64 lockstart, lockend;
-	u64 num_bytes;
-	int ret;
+	u64 cur_offset;
+	int ret = 0;
 
 	if (!(inode->flags & (BTRFS_INODE_NODATACOW | BTRFS_INODE_PREALLOC)))
 		return 0;
···
 	lockstart = round_down(pos, fs_info->sectorsize);
 	lockend = round_up(pos + *write_bytes,
 			   fs_info->sectorsize) - 1;
-	num_bytes = lockend - lockstart + 1;
 
 	if (nowait) {
 		if (!btrfs_try_lock_ordered_range(inode, lockstart, lockend,
···
 		btrfs_lock_and_flush_ordered_range(inode, lockstart, lockend,
 						   &cached_state);
 	}
-	ret = can_nocow_extent(inode, lockstart, &num_bytes, NULL, nowait);
-	if (ret <= 0)
-		btrfs_drew_write_unlock(&root->snapshot_lock);
-	else
-		*write_bytes = min_t(size_t, *write_bytes ,
-				     num_bytes - pos + lockstart);
+
+	cur_offset = lockstart;
+	while (cur_offset < lockend) {
+		u64 num_bytes = lockend - cur_offset + 1;
+
+		ret = can_nocow_extent(inode, cur_offset, &num_bytes, NULL, nowait);
+		if (ret <= 0) {
+			/*
+			 * If cur_offset == lockstart it means we haven't found
+			 * any extent against which we can NOCOW, so unlock the
+			 * snapshot lock.
+			 */
+			if (cur_offset == lockstart)
+				btrfs_drew_write_unlock(&root->snapshot_lock);
+			break;
+		}
+		cur_offset += num_bytes;
+	}
+
 	btrfs_unlock_extent(&inode->io_tree, lockstart, lockend, &cached_state);
+
+	/*
+	 * cur_offset > lockstart means there's at least a partial range we can
+	 * NOCOW, and that range can cover one or more extents.
+	 */
+	if (cur_offset > lockstart) {
+		*write_bytes = min_t(size_t, *write_bytes, cur_offset - pos);
+		return 1;
+	}
 
 	return ret;
 }