[PATCH v2 2/6] btrfs: introduce a new fs state, EMERGENCY_SHUTDOWN

[Qu Wenruo <wqu@xxxxxxxx>]

This is btrfs' equivalent of XFS_IOC_GOINGDOWN or EXT4_IOC_SHUTDOWN,
after entering the emergency shutdown state, all operations will return
errors (-EIO), and can not be bring back to normal state until unmount.

A new helper, btrfs_force_shutdown() is introduced, which will:

- Mark the fs as error
  But without flipping the fs read-only.
  This is a special handling for the future shutdown ioctl, which will
  freeze the fs first, set the SHUTDOWN flag, thaw the fs.

  But the thaw path will no longer call the unfreeze_fs() call back
  if the superblock is already read-only.

  So to handle future shutdown correctly, we only mark the fs as error,
  without flipping it read-only.

- Set the SHUTDOWN flag and output an message

New users of those interfaces will be added when implementing shutdown
ioctl support.

Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
---
 fs/btrfs/fs.h       | 28 ++++++++++++++++++++++++++++
 fs/btrfs/messages.c |  1 +
 2 files changed, 29 insertions(+)

diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
index d90304d4e32c..45185c095696 100644
--- a/fs/btrfs/fs.h
+++ b/fs/btrfs/fs.h
@@ -29,6 +29,7 @@
 #include "extent-io-tree.h"
 #include "async-thread.h"
 #include "block-rsv.h"
+#include "messages.h"
 
 struct inode;
 struct super_block;
@@ -120,6 +121,12 @@ enum {
 	/* No more delayed iput can be queued. */
 	BTRFS_FS_STATE_NO_DELAYED_IPUT,
 
+	/*
+	 * Emergency shutdown, a step further than trans aborted by rejecting
+	 * all operations.
+	 */
+	BTRFS_FS_STATE_EMERGENCY_SHUTDOWN,
+
 	BTRFS_FS_STATE_COUNT
 };
 
@@ -1100,6 +1107,27 @@ static inline void btrfs_wake_unfinished_drop(struct btrfs_fs_info *fs_info)
 	(unlikely(test_bit(BTRFS_FS_STATE_LOG_CLEANUP_ERROR,		\
 			   &(fs_info)->fs_state)))
 
+static inline bool btrfs_is_shutdown(struct btrfs_fs_info *fs_info)
+{
+	return test_bit(BTRFS_FS_STATE_EMERGENCY_SHUTDOWN, &fs_info->fs_state);
+}
+
+static inline void btrfs_force_shutdown(struct btrfs_fs_info *fs_info)
+{
+	/*
+	 * Here we do not want to use handle_fs_error(), which will mark
+	 * the fs read-only.
+	 * Some call sites like shutdown ioctl will mark the fs shutdown
+	 * when the fs is frozen. But thaw path will handle RO and RW fs
+	 * differently.
+	 *
+	 * So here we only mark the fs error without flipping it RO.
+	 */
+	WRITE_ONCE(fs_info->fs_error, -EIO);
+	if (!test_and_set_bit(BTRFS_FS_STATE_EMERGENCY_SHUTDOWN, &fs_info->fs_state))
+		btrfs_info(fs_info, "emergency shutdown");
+}
+
 /*
  * We use folio flag owner_2 to indicate there is an ordered extent with
  * unfinished IO.
diff --git a/fs/btrfs/messages.c b/fs/btrfs/messages.c
index 363fd28c0268..2bb4bcb7c2cd 100644
--- a/fs/btrfs/messages.c
+++ b/fs/btrfs/messages.c
@@ -23,6 +23,7 @@ static const char fs_state_chars[] = {
 	[BTRFS_FS_STATE_NO_DATA_CSUMS]		= 'C',
 	[BTRFS_FS_STATE_SKIP_META_CSUMS]	= 'S',
 	[BTRFS_FS_STATE_LOG_CLEANUP_ERROR]	= 'L',
+	[BTRFS_FS_STATE_EMERGENCY_SHUTDOWN]	= 'E',
 };
 
 static void btrfs_state_to_string(const struct btrfs_fs_info *info, char *buf)
-- 
2.50.0