fs/ext4/mmp.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/fs.h>
 #include <linux/random.h>
 #include <linux/buffer_head.h>
 #include <linux/utsname.h>
 #include <linux/kthread.h>

 #include "ext4.h"

 /* Checksumming functions */
 static __le32 ext4_mmp_csum(struct super_block *sb, struct mmp_struct *mmp)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	int offset = offsetof(struct mmp_struct, mmp_checksum);
 	__u32 csum;

 	csum = ext4_chksum(sbi, sbi->s_csum_seed, (char *)mmp, offset);

 	return cpu_to_le32(csum);
 }

 static int ext4_mmp_csum_verify(struct super_block *sb, struct mmp_struct *mmp)
 {
 	if (!ext4_has_metadata_csum(sb))
 		return 1;

 	return mmp->mmp_checksum == ext4_mmp_csum(sb, mmp);
 }

 static void ext4_mmp_csum_set(struct super_block *sb, struct mmp_struct *mmp)
 {
 	if (!ext4_has_metadata_csum(sb))
 		return;

 	mmp->mmp_checksum = ext4_mmp_csum(sb, mmp);
 }

 /*
  * Write the MMP block using REQ_SYNC to try to get the block on-disk
  * faster.
  */
 static int write_mmp_block_thawed(struct super_block *sb,
 				  struct buffer_head *bh)
 {
 	struct mmp_struct *mmp = (struct mmp_struct *)(bh->b_data);

 	ext4_mmp_csum_set(sb, mmp);
 	lock_buffer(bh);
 	bh->b_end_io = end_buffer_write_sync;
 	get_bh(bh);
 	submit_bh(REQ_OP_WRITE | REQ_SYNC | REQ_META | REQ_PRIO, bh);
 	wait_on_buffer(bh);
 	if (unlikely(!buffer_uptodate(bh)))
 		return -EIO;
 	return 0;
 }

 static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
 {
 	int err;

 	/*
 	 * We protect against freezing so that we don't create dirty buffers
 	 * on frozen filesystem.
 	 */
 	sb_start_write(sb);
 	err = write_mmp_block_thawed(sb, bh);
 	sb_end_write(sb);
 	return err;
 }

 /*
  * Read the MMP block. It _must_ be read from disk and hence we clear the
  * uptodate flag on the buffer.
  */
 static int read_mmp_block(struct super_block *sb, struct buffer_head **bh,
 			  ext4_fsblk_t mmp_block)
 {
 	struct mmp_struct *mmp;
 	int ret;

 	if (*bh)
 		clear_buffer_uptodate(*bh);

 	/* This would be sb_bread(sb, mmp_block), except we need to be sure
 	 * that the MD RAID device cache has been bypassed, and that the read
 	 * is not blocked in the elevator. */
 	if (!*bh) {
 		*bh = sb_getblk(sb, mmp_block);
 		if (!*bh) {
 			ret = -ENOMEM;
 			goto warn_exit;
 		}
 	}

 	lock_buffer(*bh);
 	ret = ext4_read_bh(*bh, REQ_META | REQ_PRIO, NULL, false);
 	if (ret)
 		goto warn_exit;

 	mmp = (struct mmp_struct *)((*bh)->b_data);
 	if (le32_to_cpu(mmp->mmp_magic) != EXT4_MMP_MAGIC) {
 		ret = -EFSCORRUPTED;
 		goto warn_exit;
 	}
 	if (!ext4_mmp_csum_verify(sb, mmp)) {
 		ret = -EFSBADCRC;
 		goto warn_exit;
 	}
 	return 0;
 warn_exit:
 	brelse(*bh);
 	*bh = NULL;
 	ext4_warning(sb, "Error %d while reading MMP block %llu",
 		     ret, mmp_block);
 	return ret;
 }

 /*
  * Dump as much information as possible to help the admin.
  */
 void __dump_mmp_msg(struct super_block *sb, struct mmp_struct *mmp,
 		    const char *function, unsigned int line, const char *msg)
 {
 	__ext4_warning(sb, function, line, "%s", msg);
 	__ext4_warning(sb, function, line,
 		       "MMP failure info: last update time: %llu, last update node: %.*s, last update device: %.*s",
 		       (unsigned long long)le64_to_cpu(mmp->mmp_time),
 		       (int)sizeof(mmp->mmp_nodename), mmp->mmp_nodename,
 		       (int)sizeof(mmp->mmp_bdevname), mmp->mmp_bdevname);
 }

 /*
  * kmmpd will update the MMP sequence every s_mmp_update_interval seconds
  */
 static int kmmpd(void *data)
 {
 	struct super_block *sb = data;
 	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
 	struct buffer_head *bh = EXT4_SB(sb)->s_mmp_bh;
 	struct mmp_struct *mmp;
 	ext4_fsblk_t mmp_block;
 	u32 seq = 0;
 	unsigned long failed_writes = 0;
 	int mmp_update_interval = le16_to_cpu(es->s_mmp_update_interval);
 	unsigned mmp_check_interval;
 	unsigned long last_update_time;
 	unsigned long diff;
 	int retval = 0;

 	mmp_block = le64_to_cpu(es->s_mmp_block);
 	mmp = (struct mmp_struct *)(bh->b_data);
 	mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
 	/*
 	 * Start with the higher mmp_check_interval and reduce it if
 	 * the MMP block is being updated on time.
 	 */
 	mmp_check_interval = max(EXT4_MMP_CHECK_MULT * mmp_update_interval,
 				 EXT4_MMP_MIN_CHECK_INTERVAL);
 	mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);

 	memcpy(mmp->mmp_nodename, init_utsname()->nodename,
 	       sizeof(mmp->mmp_nodename));

 	while (!kthread_should_stop() && !ext4_forced_shutdown(sb)) {
 		if (!ext4_has_feature_mmp(sb)) {
 			ext4_warning(sb, "kmmpd being stopped since MMP feature"
 				     " has been disabled.");
 			goto wait_to_exit;
 		}
 		if (++seq > EXT4_MMP_SEQ_MAX)
 			seq = 1;

 		mmp->mmp_seq = cpu_to_le32(seq);
 		mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
 		last_update_time = jiffies;

 		retval = write_mmp_block(sb, bh);
 		/*
 		 * Don't spew too many error messages. Print one every
 		 * (s_mmp_update_interval * 60) seconds.
 		 */
 		if (retval) {
 			if ((failed_writes % 60) == 0) {
 				ext4_error_err(sb, -retval,
 					       "Error writing to MMP block");
 			}
 			failed_writes++;
 		}

 		diff = jiffies - last_update_time;
 		if (diff < mmp_update_interval * HZ)
 			schedule_timeout_interruptible(mmp_update_interval *
 						       HZ - diff);

 		/*
 		 * We need to make sure that more than mmp_check_interval
 		 * seconds have not passed since writing. If that has happened
 		 * we need to check if the MMP block is as we left it.
 		 */
 		diff = jiffies - last_update_time;
 		if (diff > mmp_check_interval * HZ) {
 			struct buffer_head *bh_check = NULL;
 			struct mmp_struct *mmp_check;

 			retval = read_mmp_block(sb, &bh_check, mmp_block);
 			if (retval) {
 				ext4_error_err(sb, -retval,
 					       "error reading MMP data: %d",
 					       retval);
 				goto wait_to_exit;
 			}

 			mmp_check = (struct mmp_struct *)(bh_check->b_data);
 			if (mmp->mmp_seq != mmp_check->mmp_seq ||
 			    memcmp(mmp->mmp_nodename, mmp_check->mmp_nodename,
 				   sizeof(mmp->mmp_nodename))) {
 				dump_mmp_msg(sb, mmp_check,
 					     "Error while updating MMP info. "
 					     "The filesystem seems to have been"
 					     " multiply mounted.");
 				ext4_error_err(sb, EBUSY, "abort");
 				put_bh(bh_check);
 				retval = -EBUSY;
 				goto wait_to_exit;
 			}
 			put_bh(bh_check);
 		}

 		 /*
 		 * Adjust the mmp_check_interval depending on how much time
 		 * it took for the MMP block to be written.
 		 */
 		mmp_check_interval = max(min(EXT4_MMP_CHECK_MULT * diff / HZ,
 					     EXT4_MMP_MAX_CHECK_INTERVAL),
 					 EXT4_MMP_MIN_CHECK_INTERVAL);
 		mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);
 	}

 	/*
 	 * Unmount seems to be clean.
 	 */
 	mmp->mmp_seq = cpu_to_le32(EXT4_MMP_SEQ_CLEAN);
 	mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());

 	retval = write_mmp_block(sb, bh);

 wait_to_exit:
 	while (!kthread_should_stop()) {
 		set_current_state(TASK_INTERRUPTIBLE);
 		if (!kthread_should_stop())
 			schedule();
 	}
 	set_current_state(TASK_RUNNING);
 	return retval;
 }

 void ext4_stop_mmpd(struct ext4_sb_info *sbi)
 {
 	if (sbi->s_mmp_tsk) {
 		kthread_stop(sbi->s_mmp_tsk);
 		brelse(sbi->s_mmp_bh);
 		sbi->s_mmp_tsk = NULL;
 	}
 }

 /*
  * Get a random new sequence number but make sure it is not greater than
  * EXT4_MMP_SEQ_MAX.
  */
 static unsigned int mmp_new_seq(void)
 {
 	return get_random_u32_below(EXT4_MMP_SEQ_MAX + 1);
 }

 /*
  * Protect the filesystem from being mounted more than once.
  */
 int ext4_multi_mount_protect(struct super_block *sb,
 				    ext4_fsblk_t mmp_block)
 {
 	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
 	struct buffer_head *bh = NULL;
 	struct mmp_struct *mmp = NULL;
 	u32 seq;
 	unsigned int mmp_check_interval = le16_to_cpu(es->s_mmp_update_interval);
 	unsigned int wait_time = 0;
 	int retval;

 	if (mmp_block < le32_to_cpu(es->s_first_data_block) ||
 	    mmp_block >= ext4_blocks_count(es)) {
 		ext4_warning(sb, "Invalid MMP block in superblock");
 		retval = -EINVAL;
 		goto failed;
 	}

 	retval = read_mmp_block(sb, &bh, mmp_block);
 	if (retval)
 		goto failed;

 	mmp = (struct mmp_struct *)(bh->b_data);

 	if (mmp_check_interval < EXT4_MMP_MIN_CHECK_INTERVAL)
 		mmp_check_interval = EXT4_MMP_MIN_CHECK_INTERVAL;

 	/*
 	 * If check_interval in MMP block is larger, use that instead of
 	 * update_interval from the superblock.
 	 */
 	if (le16_to_cpu(mmp->mmp_check_interval) > mmp_check_interval)
 		mmp_check_interval = le16_to_cpu(mmp->mmp_check_interval);

 	seq = le32_to_cpu(mmp->mmp_seq);
 	if (seq == EXT4_MMP_SEQ_CLEAN)
 		goto skip;

 	if (seq == EXT4_MMP_SEQ_FSCK) {
 		dump_mmp_msg(sb, mmp, "fsck is running on the filesystem");
 		retval = -EBUSY;
 		goto failed;
 	}

 	wait_time = min(mmp_check_interval * 2 + 1,
 			mmp_check_interval + 60);

 	/* Print MMP interval if more than 20 secs. */
 	if (wait_time > EXT4_MMP_MIN_CHECK_INTERVAL * 4)
 		ext4_warning(sb, "MMP interval %u higher than expected, please"
 			     " wait.\n", wait_time * 2);

 	if (schedule_timeout_interruptible(HZ * wait_time) != 0) {
 		ext4_warning(sb, "MMP startup interrupted, failing mount\n");
 		retval = -ETIMEDOUT;
 		goto failed;
 	}

 	retval = read_mmp_block(sb, &bh, mmp_block);
 	if (retval)
 		goto failed;
 	mmp = (struct mmp_struct *)(bh->b_data);
 	if (seq != le32_to_cpu(mmp->mmp_seq)) {
 		dump_mmp_msg(sb, mmp,
 			     "Device is already active on another node.");
 		retval = -EBUSY;
 		goto failed;
 	}

 skip:
 	/*
 	 * write a new random sequence number.
 	 */
 	seq = mmp_new_seq();
 	mmp->mmp_seq = cpu_to_le32(seq);

 	/*
 	 * On mount / remount we are protected against fs freezing (by s_umount
 	 * semaphore) and grabbing freeze protection upsets lockdep
 	 */
 	retval = write_mmp_block_thawed(sb, bh);
 	if (retval)
 		goto failed;

 	/*
 	 * wait for MMP interval and check mmp_seq.
 	 */
 	if (schedule_timeout_interruptible(HZ * wait_time) != 0) {
 		ext4_warning(sb, "MMP startup interrupted, failing mount");
 		retval = -ETIMEDOUT;
 		goto failed;
 	}

 	retval = read_mmp_block(sb, &bh, mmp_block);
 	if (retval)
 		goto failed;
 	mmp = (struct mmp_struct *)(bh->b_data);
 	if (seq != le32_to_cpu(mmp->mmp_seq)) {
 		dump_mmp_msg(sb, mmp,
 			     "Device is already active on another node.");
 		retval = -EBUSY;
 		goto failed;
 	}

 	EXT4_SB(sb)->s_mmp_bh = bh;

 	BUILD_BUG_ON(sizeof(mmp->mmp_bdevname) < BDEVNAME_SIZE);
 	snprintf(mmp->mmp_bdevname, sizeof(mmp->mmp_bdevname),
 		 "%pg", bh->b_bdev);

 	/*
 	 * Start a kernel thread to update the MMP block periodically.
 	 */
 	EXT4_SB(sb)->s_mmp_tsk = kthread_run(kmmpd, sb, "kmmpd-%.*s",
 					     (int)sizeof(mmp->mmp_bdevname),
 					     mmp->mmp_bdevname);
 	if (IS_ERR(EXT4_SB(sb)->s_mmp_tsk)) {
 		EXT4_SB(sb)->s_mmp_tsk = NULL;
 		ext4_warning(sb, "Unable to create kmmpd thread for %s.",
 			     sb->s_id);
 		retval = -ENOMEM;
 		goto failed;
 	}

 	return 0;

 failed:
 	brelse(bh);
 	return retval;
 }
	// SPDX-License-Identifier: GPL-2.0
	#include <linux/fs.h>
	#include <linux/random.h>
	#include <linux/buffer_head.h>
	#include <linux/utsname.h>
	#include <linux/kthread.h>

	#include "ext4.h"

	/* Checksumming functions */
	static __le32 ext4_mmp_csum(struct super_block sb, struct mmp_struct mmp)
	{
	struct ext4_sb_info *sbi = EXT4_SB(sb);
	int offset = offsetof(struct mmp_struct, mmp_checksum);
	__u32 csum;

	csum = ext4_chksum(sbi, sbi->s_csum_seed, (char *)mmp, offset);

	return cpu_to_le32(csum);
	}

	static int ext4_mmp_csum_verify(struct super_block sb, struct mmp_struct mmp)
	{
	if (!ext4_has_metadata_csum(sb))
	return 1;

	return mmp->mmp_checksum == ext4_mmp_csum(sb, mmp);
	}

	static void ext4_mmp_csum_set(struct super_block sb, struct mmp_struct mmp)
	{
	if (!ext4_has_metadata_csum(sb))
	return;

	mmp->mmp_checksum = ext4_mmp_csum(sb, mmp);
	}

	/*
	* Write the MMP block using REQ_SYNC to try to get the block on-disk
	* faster.
	*/
	static int write_mmp_block_thawed(struct super_block *sb,
	struct buffer_head *bh)
	{
	struct mmp_struct mmp = (struct mmp_struct )(bh->b_data);

	ext4_mmp_csum_set(sb, mmp);
	lock_buffer(bh);
	bh->b_end_io = end_buffer_write_sync;
	get_bh(bh);
	submit_bh(REQ_OP_WRITE \| REQ_SYNC \| REQ_META \| REQ_PRIO, bh);
	wait_on_buffer(bh);
	if (unlikely(!buffer_uptodate(bh)))
	return -EIO;
	return 0;
	}

	static int write_mmp_block(struct super_block sb, struct buffer_head bh)
	{
	int err;

	/*
	* We protect against freezing so that we don't create dirty buffers
	* on frozen filesystem.
	*/
	sb_start_write(sb);
	err = write_mmp_block_thawed(sb, bh);
	sb_end_write(sb);
	return err;
	}

	/*
	* Read the MMP block. It _must_ be read from disk and hence we clear the
	* uptodate flag on the buffer.
	*/
	static int read_mmp_block(struct super_block sb, struct buffer_head *bh,
	ext4_fsblk_t mmp_block)
	{
	struct mmp_struct *mmp;
	int ret;

	if (*bh)
	clear_buffer_uptodate(*bh);

	/* This would be sb_bread(sb, mmp_block), except we need to be sure
	* that the MD RAID device cache has been bypassed, and that the read
	* is not blocked in the elevator. */
	if (!*bh) {
	*bh = sb_getblk(sb, mmp_block);
	if (!*bh) {
	ret = -ENOMEM;
	goto warn_exit;
	}
	}

	lock_buffer(*bh);
	ret = ext4_read_bh(*bh, REQ_META \| REQ_PRIO, NULL, false);
	if (ret)
	goto warn_exit;

	mmp = (struct mmp_struct )((bh)->b_data);
	if (le32_to_cpu(mmp->mmp_magic) != EXT4_MMP_MAGIC) {
	ret = -EFSCORRUPTED;
	goto warn_exit;
	}
	if (!ext4_mmp_csum_verify(sb, mmp)) {
	ret = -EFSBADCRC;
	goto warn_exit;
	}
	return 0;
	warn_exit:
	brelse(*bh);
	*bh = NULL;
	ext4_warning(sb, "Error %d while reading MMP block %llu",
	ret, mmp_block);
	return ret;
	}

	/*
	* Dump as much information as possible to help the admin.
	*/
	void __dump_mmp_msg(struct super_block sb, struct mmp_struct mmp,
	const char function, unsigned int line, const char msg)
	{
	__ext4_warning(sb, function, line, "%s", msg);
	__ext4_warning(sb, function, line,
	"MMP failure info: last update time: %llu, last update node: %.s, last update device: %.s",
	(unsigned long long)le64_to_cpu(mmp->mmp_time),
	(int)sizeof(mmp->mmp_nodename), mmp->mmp_nodename,
	(int)sizeof(mmp->mmp_bdevname), mmp->mmp_bdevname);
	}

	/*
	* kmmpd will update the MMP sequence every s_mmp_update_interval seconds
	*/
	static int kmmpd(void *data)
	{
	struct super_block *sb = data;
	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
	struct buffer_head *bh = EXT4_SB(sb)->s_mmp_bh;
	struct mmp_struct *mmp;
	ext4_fsblk_t mmp_block;
	u32 seq = 0;
	unsigned long failed_writes = 0;
	int mmp_update_interval = le16_to_cpu(es->s_mmp_update_interval);
	unsigned mmp_check_interval;
	unsigned long last_update_time;
	unsigned long diff;
	int retval = 0;

	mmp_block = le64_to_cpu(es->s_mmp_block);
	mmp = (struct mmp_struct *)(bh->b_data);
	mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
	/*
	* Start with the higher mmp_check_interval and reduce it if
	* the MMP block is being updated on time.
	*/
	mmp_check_interval = max(EXT4_MMP_CHECK_MULT * mmp_update_interval,
	EXT4_MMP_MIN_CHECK_INTERVAL);
	mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);

	memcpy(mmp->mmp_nodename, init_utsname()->nodename,
	sizeof(mmp->mmp_nodename));

	while (!kthread_should_stop() && !ext4_forced_shutdown(sb)) {
	if (!ext4_has_feature_mmp(sb)) {
	ext4_warning(sb, "kmmpd being stopped since MMP feature"
	" has been disabled.");
	goto wait_to_exit;
	}
	if (++seq > EXT4_MMP_SEQ_MAX)
	seq = 1;

	mmp->mmp_seq = cpu_to_le32(seq);
	mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
	last_update_time = jiffies;

	retval = write_mmp_block(sb, bh);
	/*
	* Don't spew too many error messages. Print one every
	* (s_mmp_update_interval * 60) seconds.
	*/
	if (retval) {
	if ((failed_writes % 60) == 0) {
	ext4_error_err(sb, -retval,
	"Error writing to MMP block");
	}
	failed_writes++;
	}

	diff = jiffies - last_update_time;
	if (diff < mmp_update_interval * HZ)
	schedule_timeout_interruptible(mmp_update_interval *
	HZ - diff);

	/*
	* We need to make sure that more than mmp_check_interval
	* seconds have not passed since writing. If that has happened
	* we need to check if the MMP block is as we left it.
	*/
	diff = jiffies - last_update_time;
	if (diff > mmp_check_interval * HZ) {
	struct buffer_head *bh_check = NULL;
	struct mmp_struct *mmp_check;

	retval = read_mmp_block(sb, &bh_check, mmp_block);
	if (retval) {
	ext4_error_err(sb, -retval,
	"error reading MMP data: %d",
	retval);
	goto wait_to_exit;
	}

	mmp_check = (struct mmp_struct *)(bh_check->b_data);
	if (mmp->mmp_seq != mmp_check->mmp_seq \|\|
	memcmp(mmp->mmp_nodename, mmp_check->mmp_nodename,
	sizeof(mmp->mmp_nodename))) {
	dump_mmp_msg(sb, mmp_check,
	"Error while updating MMP info. "
	"The filesystem seems to have been"
	" multiply mounted.");
	ext4_error_err(sb, EBUSY, "abort");
	put_bh(bh_check);
	retval = -EBUSY;
	goto wait_to_exit;
	}
	put_bh(bh_check);
	}

	/*
	* Adjust the mmp_check_interval depending on how much time
	* it took for the MMP block to be written.
	*/
	mmp_check_interval = max(min(EXT4_MMP_CHECK_MULT * diff / HZ,
	EXT4_MMP_MAX_CHECK_INTERVAL),
	EXT4_MMP_MIN_CHECK_INTERVAL);
	mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);
	}

	/*
	* Unmount seems to be clean.
	*/
	mmp->mmp_seq = cpu_to_le32(EXT4_MMP_SEQ_CLEAN);
	mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());

	retval = write_mmp_block(sb, bh);

	wait_to_exit:
	while (!kthread_should_stop()) {
	set_current_state(TASK_INTERRUPTIBLE);
	if (!kthread_should_stop())
	schedule();
	}
	set_current_state(TASK_RUNNING);
	return retval;
	}

	void ext4_stop_mmpd(struct ext4_sb_info *sbi)
	{
	if (sbi->s_mmp_tsk) {
	kthread_stop(sbi->s_mmp_tsk);
	brelse(sbi->s_mmp_bh);
	sbi->s_mmp_tsk = NULL;
	}
	}

	/*
	* Get a random new sequence number but make sure it is not greater than
	* EXT4_MMP_SEQ_MAX.
	*/
	static unsigned int mmp_new_seq(void)
	{
	return get_random_u32_below(EXT4_MMP_SEQ_MAX + 1);
	}

	/*
	* Protect the filesystem from being mounted more than once.
	*/
	int ext4_multi_mount_protect(struct super_block *sb,
	ext4_fsblk_t mmp_block)
	{
	struct ext4_super_block *es = EXT4_SB(sb)->s_es;
	struct buffer_head *bh = NULL;
	struct mmp_struct *mmp = NULL;
	u32 seq;
	unsigned int mmp_check_interval = le16_to_cpu(es->s_mmp_update_interval);
	unsigned int wait_time = 0;
	int retval;

	if (mmp_block < le32_to_cpu(es->s_first_data_block) \|\|
	mmp_block >= ext4_blocks_count(es)) {
	ext4_warning(sb, "Invalid MMP block in superblock");
	retval = -EINVAL;
	goto failed;
	}

	retval = read_mmp_block(sb, &bh, mmp_block);
	if (retval)
	goto failed;

	mmp = (struct mmp_struct *)(bh->b_data);

	if (mmp_check_interval < EXT4_MMP_MIN_CHECK_INTERVAL)
	mmp_check_interval = EXT4_MMP_MIN_CHECK_INTERVAL;

	/*
	* If check_interval in MMP block is larger, use that instead of
	* update_interval from the superblock.
	*/
	if (le16_to_cpu(mmp->mmp_check_interval) > mmp_check_interval)
	mmp_check_interval = le16_to_cpu(mmp->mmp_check_interval);

	seq = le32_to_cpu(mmp->mmp_seq);
	if (seq == EXT4_MMP_SEQ_CLEAN)
	goto skip;

	if (seq == EXT4_MMP_SEQ_FSCK) {
	dump_mmp_msg(sb, mmp, "fsck is running on the filesystem");
	retval = -EBUSY;
	goto failed;
	}

	wait_time = min(mmp_check_interval * 2 + 1,
	mmp_check_interval + 60);

	/* Print MMP interval if more than 20 secs. */
	if (wait_time > EXT4_MMP_MIN_CHECK_INTERVAL * 4)
	ext4_warning(sb, "MMP interval %u higher than expected, please"
	" wait.\n", wait_time * 2);

	if (schedule_timeout_interruptible(HZ * wait_time) != 0) {
	ext4_warning(sb, "MMP startup interrupted, failing mount\n");
	retval = -ETIMEDOUT;
	goto failed;
	}

	retval = read_mmp_block(sb, &bh, mmp_block);
	if (retval)
	goto failed;
	mmp = (struct mmp_struct *)(bh->b_data);
	if (seq != le32_to_cpu(mmp->mmp_seq)) {
	dump_mmp_msg(sb, mmp,
	"Device is already active on another node.");
	retval = -EBUSY;
	goto failed;
	}

	skip:
	/*
	* write a new random sequence number.
	*/
	seq = mmp_new_seq();
	mmp->mmp_seq = cpu_to_le32(seq);

	/*
	* On mount / remount we are protected against fs freezing (by s_umount
	* semaphore) and grabbing freeze protection upsets lockdep
	*/
	retval = write_mmp_block_thawed(sb, bh);
	if (retval)
	goto failed;

	/*
	* wait for MMP interval and check mmp_seq.
	*/
	if (schedule_timeout_interruptible(HZ * wait_time) != 0) {
	ext4_warning(sb, "MMP startup interrupted, failing mount");
	retval = -ETIMEDOUT;
	goto failed;
	}

	retval = read_mmp_block(sb, &bh, mmp_block);
	if (retval)
	goto failed;
	mmp = (struct mmp_struct *)(bh->b_data);
	if (seq != le32_to_cpu(mmp->mmp_seq)) {
	dump_mmp_msg(sb, mmp,
	"Device is already active on another node.");
	retval = -EBUSY;
	goto failed;
	}

	EXT4_SB(sb)->s_mmp_bh = bh;

	BUILD_BUG_ON(sizeof(mmp->mmp_bdevname) < BDEVNAME_SIZE);
	snprintf(mmp->mmp_bdevname, sizeof(mmp->mmp_bdevname),
	"%pg", bh->b_bdev);

	/*
	* Start a kernel thread to update the MMP block periodically.
	*/
	EXT4_SB(sb)->s_mmp_tsk = kthread_run(kmmpd, sb, "kmmpd-%.*s",
	(int)sizeof(mmp->mmp_bdevname),
	mmp->mmp_bdevname);
	if (IS_ERR(EXT4_SB(sb)->s_mmp_tsk)) {
	EXT4_SB(sb)->s_mmp_tsk = NULL;
	ext4_warning(sb, "Unable to create kmmpd thread for %s.",
	sb->s_id);
	retval = -ENOMEM;
	goto failed;
	}

	return 0;

	failed:
	brelse(bh);
	return retval;
	}