btrfs: factor btrfs_check_rw_degradable() to check given device

Update btrfs_check_rw_degradable() to check against the given device if
its lost.

We can use this function to know if the volume is going to be in
degraded mode OR failed state, when the given device fails.  Which is
needed when we are handling the device failed state.

A preparatory patch does not affect the flow as such.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
[ enhance comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -2866,7 +2866,7 @@ int open_ctree(struct super_block *sb,
 		goto fail_sysfs;
 	}
 
-	if (!sb_rdonly(sb) && !btrfs_check_rw_degradable(fs_info)) {
+	if (!sb_rdonly(sb) && !btrfs_check_rw_degradable(fs_info, NULL)) {
 		btrfs_warn(fs_info,
 		"writeable mount is not allowed due to too many missing devices");
 		goto fail_sysfs;
@@ -3369,7 +3369,7 @@ static blk_status_t wait_dev_flush(struct btrfs_device *device)
 
 static int check_barrier_error(struct btrfs_fs_info *fs_info)
 {
-	if (!btrfs_check_rw_degradable(fs_info))
+	if (!btrfs_check_rw_degradable(fs_info, NULL))
 		return -EIO;
 	return 0;
 }

fs/btrfs/super.c

@@ -1844,7 +1844,7 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 			goto restore;
 		}
 
-		if (!btrfs_check_rw_degradable(fs_info)) {
+		if (!btrfs_check_rw_degradable(fs_info, NULL)) {
 			btrfs_warn(fs_info,
 				"too many missing devices, writeable remount is not allowed");
 			ret = -EACCES;

fs/btrfs/volumes.c

@@ -6851,10 +6851,13 @@ int btrfs_read_sys_array(struct btrfs_fs_info *fs_info)
 /*
  * Check if all chunks in the fs are OK for read-write degraded mount
  *
+ * If the @failing_dev is specified, it's accounted as missing.
+ *
  * Return true if all chunks meet the minimal RW mount requirements.
  * Return false if any chunk doesn't meet the minimal RW mount requirements.
  */
-bool btrfs_check_rw_degradable(struct btrfs_fs_info *fs_info)
+bool btrfs_check_rw_degradable(struct btrfs_fs_info *fs_info,
+					struct btrfs_device *failing_dev)
 {
 	struct btrfs_mapping_tree *map_tree = &fs_info->mapping_tree;
 	struct extent_map *em;
@@ -6886,8 +6889,11 @@ bool btrfs_check_rw_degradable(struct btrfs_fs_info *fs_info)
 			    test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state) ||
 			    dev->last_flush_error)
 				missing++;
+			else if (failing_dev && failing_dev == dev)
+				missing++;
 		}
 		if (missing > max_tolerated) {
+			if (!failing_dev)
 				btrfs_warn(fs_info,
 	"chunk %llu missing %d devices, max tolerance is %d for writeable mount",
 				   em->start, missing, max_tolerated);

fs/btrfs/volumes.h

@@ -554,7 +554,7 @@ void btrfs_update_commit_device_bytes_used(struct btrfs_fs_info *fs_info,
 struct list_head *btrfs_get_fs_uuids(void);
 void btrfs_set_fs_info_ptr(struct btrfs_fs_info *fs_info);
 void btrfs_reset_fs_info_ptr(struct btrfs_fs_info *fs_info);
-
-bool btrfs_check_rw_degradable(struct btrfs_fs_info *fs_info);
+bool btrfs_check_rw_degradable(struct btrfs_fs_info *fs_info,
+					struct btrfs_device *failing_dev);
 
 #endif