[PATCH v6 0/5] btrfs: add remove_bdev() callback

[PATCH v6 0/5] btrfs: add remove_bdev() callback

[Qu Wenruo]

[CHANGELOG]
v6:
- Rebased to the latest for-next branch
  Only minor conflicts that can be solved by git.

  The VFS patch is already merged in v6.17, thus no more needed in the
  series.

v5:
- Split remove_bdev() from shutdown()
  Now remove_bdev() will have a return value to indicate if the fs can
  handle the removal of the device.
  And if not, a non-zero (normally minus) value is returned.

  In that case ->shutdown() will be called as usual.

  This allows us to avoid unnecessary operations that only make sense
  for shutdown case, like shrinking the cache.

  This also means no change to any of the existing filesystems.

- Implement ->shutdown() callback for btrfs
  Since ->shutdown() and ->remove_bdev() call backs are separate now,
  btrfs needs to implement both.

v4:
- Update the commit message of the first patch
  Remove the out-of-date comments about the old *_shutdown() names.

v3:
- Also rename the callback functions inside each fs to *_remove_bdev()
  To keep the consistency between the interface and implementation.

- Add extra handling if the to-be-removed device is already missing in
  btrfs
  I do not know if a device can be double-removed, but the handling
  inside btrfs is pretty simple, if the target device is already
  missing, nothing needs to be done and can exit immediately.

v2:
- Enhance and rename shutdown() callback
  Rename it to remove_bdev() and add a @bdev parameter.
  For the existing call backs in filesystems, keep their callback
  function names, now something like ".remove_bdev = ext4_shutdown,"
  will be a quick indicator of the behavior.

- Remove the @surprise parameter for the remove_bdev() parameter.
  The fs_bdev_mark_dead() is already trying to sync the fs if it's not
  a surprise removal.
  So there isn't much a filesystem can do with the @surprise parameter.

- Fix btrfs error handling when the devices are not opened
  There are several cases that the fs_devices is not opened, including:
  * sget_fc() failure
  * an existing super block is returned
  * a new super block is returned but btrfS_open_fs_devices() failed

  Handle the error properly so that fs_devices is not freed twice.

RFC->v1:
- Add a new remove_bdev() callback
  Thanks all the feedback from Christian, Christoph and Jan on this new
  name.

- Add a @surprise parameter to the remove_bdev() callback
  To keep it the same as the bdev_mark_dead().

- Hide the shutdown ioctl and remove_bdev callback behind experimental 
  With the shutdown ioctl, there are at least 2 test failures (g/388, g/508).

  G/388 is related to the error handling with COW fixup.
  G/508 looks like something related to log replay.

  And the remove_bdev() doesn't have any btrfs specific test case yet to
  check the auto-degraded behavior, nor the auto-degraded behavior is
  fully discussed.

  So hide both of them behind experimental features.

- Do not use btrfs_handle_fs_error() to avoid freeze/thaw behavior change
  btrfs_handle_fs_error() will flips the fs read-only, which will
  affect freeze/thaw behavior.
  And no other fs set the fs read-only when shutting down, so follow the
  other fs to have a more consistent behavior.


Qu Wenruo (5):
  btrfs: introduce a new fs state, EMERGENCY_SHUTDOWN
  btrfs: reject file operations if in shutdown state
  btrfs: reject delalloc ranges if in shutdown state
  btrfs: implement shutdown ioctl
  btrfs: implement remove_bdev and shutdown super operation callbacks

 fs/btrfs/file.c            | 25 ++++++++++++++-
 fs/btrfs/fs.h              | 28 ++++++++++++++++
 fs/btrfs/inode.c           | 14 +++++++-
 fs/btrfs/ioctl.c           | 44 +++++++++++++++++++++++++
 fs/btrfs/messages.c        |  1 +
 fs/btrfs/reflink.c         |  3 ++
 fs/btrfs/super.c           | 66 ++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.c         |  2 ++
 fs/btrfs/volumes.h         |  5 +++
 include/uapi/linux/btrfs.h |  9 ++++++
 10 files changed, 195 insertions(+), 2 deletions(-)

-- 
2.50.1

[PATCH v6 1/5] btrfs: introduce a new fs state, EMERGENCY_SHUTDOWN

[Qu Wenruo]

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@suse.com>
---
 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 814bbc9417d2..777da6526bb2 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;
@@ -124,6 +125,12 @@ 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
 };
 
@@ -1120,6 +1127,27 @@ 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 a0cf8effe008..2f853de44473 100644
--- a/fs/btrfs/messages.c
+++ b/fs/btrfs/messages.c
@@ -24,6 +24,7 @@ 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)
-- 
2.50.1

[PATCH v6 2/5] btrfs: reject file operations if in shutdown state

[Qu Wenruo]

This includes the following callbacks of file_operations:

- read_iter()
- write_iter()
- mmap()
- open()
- remap_file_range()
- uring_cmd()
- splice_read()
  This requires a small wrapper to do the extra shutdown check, then call
  the regular filemap_splice_read() function

This should reject most of the file operations on a shutdown btrfs.

The callback ioctl() is intentionally skipped, as ext4 doesn't do the
shutdown check on ioctl() either, thus I believe there is some special
require for ioctl() callback even if the fs is fully shutdown.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/file.c    | 25 ++++++++++++++++++++++++-
 fs/btrfs/ioctl.c   |  3 +++
 fs/btrfs/reflink.c |  3 +++
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index dcbd038d1ad1..837da30303fd 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1440,6 +1440,8 @@ ssize_t btrfs_do_write_iter(struct kiocb *iocb, struct iov_iter *from,
 	struct btrfs_inode *inode = BTRFS_I(file_inode(file));
 	ssize_t num_written, num_sync;
 
+	if (unlikely(btrfs_is_shutdown(inode->root->fs_info)))
+		return -EIO;
 	/*
 	 * If the fs flips readonly due to some impossible error, although we
 	 * have opened a file as writable, we have to stop this write operation
@@ -2042,6 +2044,8 @@ static int btrfs_file_mmap_prepare(struct vm_area_desc *desc)
 	struct file *filp = desc->file;
 	struct address_space *mapping = filp->f_mapping;
 
+	if (unlikely(btrfs_is_shutdown(inode_to_fs_info(file_inode(filp)))))
+		return -EIO;
 	if (!mapping->a_ops->read_folio)
 		return -ENOEXEC;
 
@@ -3101,6 +3105,9 @@ static long btrfs_fallocate(struct file *file, int mode,
 	int blocksize = BTRFS_I(inode)->root->fs_info->sectorsize;
 	int ret;
 
+	if (unlikely(btrfs_is_shutdown(inode_to_fs_info(inode))))
+		return -EIO;
+
 	/* Do not allow fallocate in ZONED mode */
 	if (btrfs_is_zoned(inode_to_fs_info(inode)))
 		return -EOPNOTSUPP;
@@ -3792,6 +3799,9 @@ static int btrfs_file_open(struct inode *inode, struct file *filp)
 {
 	int ret;
 
+	if (unlikely(btrfs_is_shutdown(inode_to_fs_info(inode))))
+		return -EIO;
+
 	filp->f_mode |= FMODE_NOWAIT | FMODE_CAN_ODIRECT;
 
 	ret = fsverity_file_open(inode, filp);
@@ -3804,6 +3814,9 @@ static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
 	ssize_t ret = 0;
 
+	if (unlikely(btrfs_is_shutdown(inode_to_fs_info(file_inode(iocb->ki_filp)))))
+		return -EIO;
+
 	if (iocb->ki_flags & IOCB_DIRECT) {
 		ret = btrfs_direct_read(iocb, to);
 		if (ret < 0 || !iov_iter_count(to) ||
@@ -3814,10 +3827,20 @@ static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	return filemap_read(iocb, to, ret);
 }
 
+static ssize_t btrfs_file_splice_read(struct file *in, loff_t *ppos,
+				      struct pipe_inode_info *pipe,
+				      size_t len, unsigned int flags)
+{
+	if (unlikely(btrfs_is_shutdown(inode_to_fs_info(file_inode(in)))))
+		return -EIO;
+
+	return filemap_splice_read(in, ppos, pipe, len, flags);
+}
+
 const struct file_operations btrfs_file_operations = {
 	.llseek		= btrfs_file_llseek,
 	.read_iter      = btrfs_file_read_iter,
-	.splice_read	= filemap_splice_read,
+	.splice_read	= btrfs_file_splice_read,
 	.write_iter	= btrfs_file_write_iter,
 	.splice_write	= iter_file_splice_write,
 	.mmap_prepare	= btrfs_file_mmap_prepare,
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 938286bee6a8..342c8412c859 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -5077,6 +5077,9 @@ static int btrfs_uring_encoded_write(struct io_uring_cmd *cmd, unsigned int issu
 
 int btrfs_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 {
+	if (unlikely(btrfs_is_shutdown(inode_to_fs_info(file_inode(cmd->file)))))
+		return -EIO;
+
 	switch (cmd->cmd_op) {
 	case BTRFS_IOC_ENCODED_READ:
 #if defined(CONFIG_64BIT) && defined(CONFIG_COMPAT)
diff --git a/fs/btrfs/reflink.c b/fs/btrfs/reflink.c
index 5465a5eae9b2..1bbe3bb7e1bb 100644
--- a/fs/btrfs/reflink.c
+++ b/fs/btrfs/reflink.c
@@ -868,6 +868,9 @@ loff_t btrfs_remap_file_range(struct file *src_file, loff_t off,
 	bool same_inode = dst_inode == src_inode;
 	int ret;
 
+	if (unlikely(btrfs_is_shutdown(inode_to_fs_info(file_inode(src_file)))))
+		return -EIO;
+
 	if (remap_flags & ~(REMAP_FILE_DEDUP | REMAP_FILE_ADVISORY))
 		return -EINVAL;
 
-- 
2.50.1

[PATCH v6 3/5] btrfs: reject delalloc ranges if in shutdown state

[Qu Wenruo]

If the filesystem has dirty pages before the fs is shutdown, we should
no longer write them back, instead should treat them as writeback error.

Handle such situation by marking all those delalloc range as error and
let error handling path to clean them up.

For ranges that already have ordered extent created, let them continue
the writeback, and at ordered io finish time the file extent item update
will be rejected as the fs is already marked error.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/inode.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index ac2fd589697d..509309edcf5d 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -871,6 +871,9 @@ static void compress_file_range(struct btrfs_work *work)
 	int compress_type = fs_info->compress_type;
 	int compress_level = fs_info->compress_level;
 
+	if (unlikely(btrfs_is_shutdown(fs_info)))
+		goto cleanup_and_bail_uncompressed;
+
 	inode_should_defrag(inode, start, end, end - start + 1, SZ_16K);
 
 	/*
@@ -1286,6 +1289,11 @@ static noinline int cow_file_range(struct btrfs_inode *inode,
 	unsigned long page_ops;
 	int ret = 0;
 
+	if (unlikely(btrfs_is_shutdown(fs_info))) {
+		ret = -EIO;
+		goto out_unlock;
+	}
+
 	if (btrfs_is_free_space_inode(inode)) {
 		ret = -EINVAL;
 		goto out_unlock;
@@ -2004,7 +2012,7 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
 {
 	struct btrfs_fs_info *fs_info = inode->root->fs_info;
 	struct btrfs_root *root = inode->root;
-	struct btrfs_path *path;
+	struct btrfs_path *path = NULL;
 	u64 cow_start = (u64)-1;
 	/*
 	 * If not 0, represents the inclusive end of the last fallback_to_cow()
@@ -2034,6 +2042,10 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
 	 */
 	ASSERT(!btrfs_is_zoned(fs_info) || btrfs_is_data_reloc_root(root));
 
+	if (unlikely(btrfs_is_shutdown(fs_info))) {
+		ret = -EIO;
+		goto error;
+	}
 	path = btrfs_alloc_path();
 	if (!path) {
 		ret = -ENOMEM;
-- 
2.50.1

[PATCH v6 4/5] btrfs: implement shutdown ioctl

[Qu Wenruo]

The shutdown ioctl should follow the XFS one, which use magic number 'X',
and ioctl number 125, with a uint32 as flags.

For now btrfs don't distinguish DEFAULT and LOGFLUSH flags (just like
f2fs), both will freeze the fs first (implies committing the current
transaction), setting the SHUTDOWN flag and finally thaw the fs.

For NOLOGFLUSH flag, the freeze/thaw part is skipped thus the current
transaction is aborted.

The new shutdown ioctl is hidden behind experimental features for more
testing.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/ioctl.c           | 41 ++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/btrfs.h |  9 +++++++++
 2 files changed, 50 insertions(+)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 342c8412c859..80dffe5780b0 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -5223,6 +5223,43 @@ static int btrfs_ioctl_subvol_sync(struct btrfs_fs_info *fs_info, void __user *a
 	return 0;
 }
 
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+static int btrfs_ioctl_shutdown(struct btrfs_fs_info *fs_info, unsigned long arg)
+{
+	int ret = 0;
+	uint32_t flags;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (get_user(flags, (uint32_t __user *)arg))
+		return -EFAULT;
+
+	if (flags >= BTRFS_SHUTDOWN_FLAGS_LAST)
+		return -EINVAL;
+
+	if (btrfs_is_shutdown(fs_info))
+		return 0;
+
+	switch (flags) {
+	case BTRFS_SHUTDOWN_FLAGS_LOGFLUSH:
+	case BTRFS_SHUTDOWN_FLAGS_DEFAULT:
+		ret = freeze_super(fs_info->sb, FREEZE_HOLDER_KERNEL, NULL);
+		if (ret)
+			return ret;
+		btrfs_force_shutdown(fs_info);
+		ret = thaw_super(fs_info->sb, FREEZE_HOLDER_KERNEL, NULL);
+		if (ret)
+			return ret;
+		break;
+	case BTRFS_SHUTDOWN_FLAGS_NOLOGFLUSH:
+		btrfs_force_shutdown(fs_info);
+		break;
+	}
+	return ret;
+}
+#endif
+
 long btrfs_ioctl(struct file *file, unsigned int
 		cmd, unsigned long arg)
 {
@@ -5378,6 +5415,10 @@ long btrfs_ioctl(struct file *file, unsigned int
 #endif
 	case BTRFS_IOC_SUBVOL_SYNC_WAIT:
 		return btrfs_ioctl_subvol_sync(fs_info, argp);
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+	case BTRFS_IOC_SHUTDOWN:
+		return btrfs_ioctl_shutdown(fs_info, arg);
+#endif
 	}
 
 	return -ENOTTY;
diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index 8e710bbb688e..f40b300bd664 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h
@@ -1099,6 +1099,12 @@ enum btrfs_err_code {
 	BTRFS_ERROR_DEV_RAID1C4_MIN_NOT_MET,
 };
 
+/* Flags for IOC_SHUTDOWN, should match XFS' flags. */
+#define BTRFS_SHUTDOWN_FLAGS_DEFAULT	0x0
+#define BTRFS_SHUTDOWN_FLAGS_LOGFLUSH	0x1
+#define BTRFS_SHUTDOWN_FLAGS_NOLOGFLUSH	0x2
+#define BTRFS_SHUTDOWN_FLAGS_LAST	0x3
+
 #define BTRFS_IOC_SNAP_CREATE _IOW(BTRFS_IOCTL_MAGIC, 1, \
 				   struct btrfs_ioctl_vol_args)
 #define BTRFS_IOC_DEFRAG _IOW(BTRFS_IOCTL_MAGIC, 2, \
@@ -1220,6 +1226,9 @@ enum btrfs_err_code {
 #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
 					struct btrfs_ioctl_subvol_wait)
 
+/* Shutdown ioctl should follow XFS's interfaces, thus not using btrfs magic. */
+#define BTRFS_IOC_SHUTDOWN	_IOR('X', 125, uint32_t)
+
 #ifdef __cplusplus
 }
 #endif
-- 
2.50.1

[PATCH v6 5/5] btrfs: implement remove_bdev and shutdown super operation callbacks

[Qu Wenruo]

For the ->remove_bdev() callback, btrfs will:

- Mark the target device as missing

- Go degraded if the fs can afford it

- Return error other wise
  Thus falls back to the shutdown callback

For the ->shutdown callback, btrfs will:

- Set the SHUTDOWN flag
  Which will reject all new incoming operations, and make all writeback
  to fail.

  The behavior is the same as the NOLOGFLUSH behavior.

To support the lookup from bdev to a btrfs_device,
btrfs_dev_lookup_args is enhanced to have a new @devt member.
If set, we should be able to use that @devt member to uniquely locating a
btrfs device.

I know the shutdown can be a little overkilled, if one has a RAID1
metadata and RAID0 data, in that case one can still read data with 50%
chance to got some good data.

But a filesystem returning -EIO for half of the time is not really
considered usable.
Further it can also be as bad as the only device went missing for a single
device btrfs.

So here we go safe other than sorry when handling missing device.

And the remove_bdev callback will be hidden behind experimental features
for now, the reasons are:

- There are not enough btrfs specific bdev removal test cases
  The existing test cases are all removing the only device, thus only
  exercises the ->shutdown() behavior.

- Not yet determined what's the expected behavior
  Although the current auto-degrade behavior is no worse than the old
  behavior, it may not always be what the end users want.

  Before there is a concrete interface, better hide the new feature
  from end users.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/super.c   | 66 ++++++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.c |  2 ++
 fs/btrfs/volumes.h |  5 ++++
 3 files changed, 73 insertions(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index aadc02374b2a..b02ab17077af 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2424,6 +2424,68 @@ static long btrfs_free_cached_objects(struct super_block *sb, struct shrink_cont
 	return 0;
 }
 
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+static int btrfs_remove_bdev(struct super_block *sb, struct block_device *bdev)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
+	struct btrfs_device *device;
+	struct btrfs_dev_lookup_args lookup_args = { .devt = bdev->bd_dev };
+	bool can_rw;
+
+	mutex_lock(&fs_info->fs_devices->device_list_mutex);
+	device = btrfs_find_device(fs_info->fs_devices, &lookup_args);
+	if (!device) {
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		/* Device not found, should not affect the running fs, just give a warning. */
+		btrfs_warn(fs_info, "unable to find btrfs device for block device '%pg'",
+			   bdev);
+		return 0;
+	}
+	/*
+	 * The to-be-removed device is already missing?
+	 *
+	 * That's weird but no special handling needed and can exit right now.
+	 */
+	if (unlikely(test_and_set_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state))) {
+		mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+		btrfs_warn(fs_info, "btrfs device id %llu is already missing",
+			   device->devid);
+		return 0;
+	}
+
+	device->fs_devices->missing_devices++;
+	if (test_and_clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state)) {
+		list_del_init(&device->dev_alloc_list);
+		WARN_ON(device->fs_devices->rw_devices < 1);
+		device->fs_devices->rw_devices--;
+	}
+	can_rw = btrfs_check_rw_degradable(fs_info, device);
+	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
+	/*
+	 * Now device is considered missing, btrfs_device_name() won't give a
+	 * meaningful result anymore, so only output the devid.
+	 */
+	if (!can_rw) {
+		btrfs_crit(fs_info,
+		"btrfs device id %llu has gone missing, can not maintain read-write",
+			   device->devid);
+		return -EIO;
+	}
+	btrfs_warn(fs_info,
+		   "btrfs device id %llu has gone missing, continue as degraded",
+		   device->devid);
+	btrfs_set_opt(fs_info->mount_opt, DEGRADED);
+	return 0;
+}
+
+static void btrfs_shutdown(struct super_block *sb)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
+
+	btrfs_force_shutdown(fs_info);
+}
+#endif
+
 static const struct super_operations btrfs_super_ops = {
 	.drop_inode	= btrfs_drop_inode,
 	.evict_inode	= btrfs_evict_inode,
@@ -2439,6 +2501,10 @@ static const struct super_operations btrfs_super_ops = {
 	.unfreeze_fs	= btrfs_unfreeze,
 	.nr_cached_objects = btrfs_nr_cached_objects,
 	.free_cached_objects = btrfs_free_cached_objects,
+#ifdef CONFIG_BTRFS_EXPERIMENTAL
+	.remove_bdev	= btrfs_remove_bdev,
+	.shutdown	= btrfs_shutdown,
+#endif
 };
 
 static const struct file_operations btrfs_ctl_fops = {
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 65b02a93db31..928fc6a061b6 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6802,6 +6802,8 @@ static bool dev_args_match_fs_devices(const struct btrfs_dev_lookup_args *args,
 static bool dev_args_match_device(const struct btrfs_dev_lookup_args *args,
 				  const struct btrfs_device *device)
 {
+	if (args->devt)
+		return device->devt == args->devt;
 	if (args->missing) {
 		if (test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state) &&
 		    !device->bdev)
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 2cbf8080eade..adbd9e6c09ff 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -662,6 +662,11 @@ struct btrfs_dev_lookup_args {
 	u64 devid;
 	u8 *uuid;
 	u8 *fsid;
+	/*
+	 * If devt is specified, all other members will be ignored as it is
+	 * enough to uniquely locate a device.
+	 */
+	dev_t devt;
 	bool missing;
 };
 
-- 
2.50.1

Re: [PATCH v6 1/5] btrfs: introduce a new fs state, EMERGENCY_SHUTDOWN

[Johannes Thumshirn]

On 10/10/25 7:01 AM, Qu Wenruo wrote:
> 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.


I think it would be better to have these helpers introduced in the 
patches that make use of them.


I.e. btrfs_is_shutdown() in 2/5 and so on.

Re: [PATCH v6 2/5] btrfs: reject file operations if in shutdown state

[Johannes Thumshirn]

Apart from the nitpick on btrfs_is_shutdown() in 1/5,

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Re: [PATCH v6 3/5] btrfs: reject delalloc ranges if in shutdown state

[Johannes Thumshirn]

On 10/10/25 7:01 AM, Qu Wenruo wrote:
> @@ -1286,6 +1289,11 @@ static noinline int cow_file_range(struct btrfs_inode *inode,
>   	unsigned long page_ops;
>   	int ret = 0;
>   
> +	if (unlikely(btrfs_is_shutdown(fs_info))) {
> +		ret = -EIO;
> +		goto out_unlock;
> +	}
> +

See below.


>   	if (btrfs_is_free_space_inode(inode)) {
>   		ret = -EINVAL;
>   		goto out_unlock;
> @@ -2004,7 +2012,7 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
>   {
>   	struct btrfs_fs_info *fs_info = inode->root->fs_info;
>   	struct btrfs_root *root = inode->root;
> -	struct btrfs_path *path;
> +	struct btrfs_path *path = NULL;
>   	u64 cow_start = (u64)-1;
>   	/*
>   	 * If not 0, represents the inclusive end of the last fallback_to_cow()
> @@ -2034,6 +2042,10 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
>   	 */
>   	ASSERT(!btrfs_is_zoned(fs_info) || btrfs_is_data_reloc_root(root));
>   
> +	if (unlikely(btrfs_is_shutdown(fs_info))) {
> +		ret = -EIO;
> +		goto error;
> +	}
>   	path = btrfs_alloc_path();
>   	if (!path) {
>   		ret = -ENOMEM;

Stupid question, going to error here will give us a error print 
(run_delalloc_nowcow failed, root= ... ). Is this intentional or would 
bailing out before allocating a path and erroring make more sense?

That on the other hand would skip the cleanup, but if needed we can 
still extract them into a cleanup function and call them before return.

Re: [PATCH v6 4/5] btrfs: implement shutdown ioctl

[Johannes Thumshirn]

On 10/10/25 7:01 AM, Qu Wenruo wrote:
> @@ -1220,6 +1226,9 @@ enum btrfs_err_code {
>   #define BTRFS_IOC_SUBVOL_SYNC_WAIT _IOW(BTRFS_IOCTL_MAGIC, 65, \
>   					struct btrfs_ioctl_subvol_wait)
>   
> +/* Shutdown ioctl should follow XFS's interfaces, thus not using btrfs magic. */
> +#define BTRFS_IOC_SHUTDOWN	_IOR('X', 125, uint32_t)
> +

Sounds like we want to have something in VFS for shutdown.


Other than that,

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Re: [PATCH v6 3/5] btrfs: reject delalloc ranges if in shutdown state

[Qu Wenruo]

在 2025/10/10 17:24, Johannes Thumshirn 写道:
> On 10/10/25 7:01 AM, Qu Wenruo wrote:
>> @@ -1286,6 +1289,11 @@ static noinline int cow_file_range(struct btrfs_inode *inode,
>>    	unsigned long page_ops;
>>    	int ret = 0;
>>    
>> +	if (unlikely(btrfs_is_shutdown(fs_info))) {
>> +		ret = -EIO;
>> +		goto out_unlock;
>> +	}
>> +
> 
> See below.
> 
> 
>>    	if (btrfs_is_free_space_inode(inode)) {
>>    		ret = -EINVAL;
>>    		goto out_unlock;
>> @@ -2004,7 +2012,7 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
>>    {
>>    	struct btrfs_fs_info *fs_info = inode->root->fs_info;
>>    	struct btrfs_root *root = inode->root;
>> -	struct btrfs_path *path;
>> +	struct btrfs_path *path = NULL;
>>    	u64 cow_start = (u64)-1;
>>    	/*
>>    	 * If not 0, represents the inclusive end of the last fallback_to_cow()
>> @@ -2034,6 +2042,10 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
>>    	 */
>>    	ASSERT(!btrfs_is_zoned(fs_info) || btrfs_is_data_reloc_root(root));
>>    
>> +	if (unlikely(btrfs_is_shutdown(fs_info))) {
>> +		ret = -EIO;
>> +		goto error;
>> +	}
>>    	path = btrfs_alloc_path();
>>    	if (!path) {
>>    		ret = -ENOMEM;
> 
> Stupid question, going to error here will give us a error print
> (run_delalloc_nowcow failed, root= ... ). Is this intentional or would
> bailing out before allocating a path and erroring make more sense?

That error message is mostly to help debugging various leakage related 
to ordered extents.

In this shutdown case, such error message will still help, because it's 
no different than an injected error.
If later we hit some ordered extent related bugs (including the folio's 
ordered flag), such error message will show the possible involved paths.

Thanks,
Qu

> 
> That on the other hand would skip the cleanup, but if needed we can
> still extract them into a cleanup function and call them before return.
> 

Re: [PATCH v6 1/5] btrfs: introduce a new fs state, EMERGENCY_SHUTDOWN

[Qu Wenruo]

在 2025/10/10 17:00, Johannes Thumshirn 写道:
> On 10/10/25 7:01 AM, Qu Wenruo wrote:
>> 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.
> 
> 
> I think it would be better to have these helpers introduced in the
> patches that make use of them.
> 
> 
> I.e. btrfs_is_shutdown() in 2/5 and so on.
> 

Indeed, the first two patches are very small, merging them together 
makes more sense.

Will go that path in the next update.

Thanks,
Qu

Re: [PATCH v6 3/5] btrfs: reject delalloc ranges if in shutdown state

[Johannes Thumshirn]

On 10/10/25 9:12 AM, Qu Wenruo wrote:
>
> 在 2025/10/10 17:24, Johannes Thumshirn 写道:
>
>> Stupid question, going to error here will give us a error print
>> (run_delalloc_nowcow failed, root= ... ). Is this intentional or would
>> bailing out before allocating a path and erroring make more sense?
> That error message is mostly to help debugging various leakage related
> to ordered extents.
>
> In this shutdown case, such error message will still help, because it's
> no different than an injected error.
> If later we hit some ordered extent related bugs (including the folio's
> ordered flag), such error message will show the possible involved paths.
>
My concern here is, that we will get user reports because of the error 
messages. If it is a debug only message, it should be btrfs_debug().

Re: [PATCH v6 3/5] btrfs: reject delalloc ranges if in shutdown state

[Qu Wenruo]

在 2025/10/10 17:59, Johannes Thumshirn 写道:
> On 10/10/25 9:12 AM, Qu Wenruo wrote:
>>
>> 在 2025/10/10 17:24, Johannes Thumshirn 写道:
>>
>>> Stupid question, going to error here will give us a error print
>>> (run_delalloc_nowcow failed, root= ... ). Is this intentional or would
>>> bailing out before allocating a path and erroring make more sense?
>> That error message is mostly to help debugging various leakage related
>> to ordered extents.
>>
>> In this shutdown case, such error message will still help, because it's
>> no different than an injected error.
>> If later we hit some ordered extent related bugs (including the folio's
>> ordered flag), such error message will show the possible involved paths.
>>
> My concern here is, that we will get user reports because of the error
> messages. If it is a debug only message, it should be btrfs_debug().

To be honest, btrfs_debug() is useless nowadays. It needs one to 
reproduce the bug in the first place, then enable the debug message.


And concerning user reports, if the fs is already shutting down, they 
will have more urgent problem to bother other than an error message in 
the dmesg.
After all, all IO, including reading cached pages, will return -EIO.

Finally there will be a message line showing "emergency shutdown", and 
we can explain to such sysadmin level users very easily.


The seemingly "debug" error message has already helped us to fix several 
hard-to-reproduce delalloc error handling bugs.

Considering how hard those errors are going to hit and how useful those 
messages are, I really prefer to keep them as is.

Thanks,
Qu