FREEZE_SUPER(9) 						   The Linux VFS						   FREEZE_SUPER(9)

NAME

       freeze_super - lock the filesystem and force it into a consistent state

SYNOPSIS

       int freeze_super(struct super_block * sb);

ARGUMENTS

       sb
	   the super to lock

DESCRIPTION

       Syncs the super to make sure the filesystem is consistent and calls the fs's freeze_fs. Subsequent calls to this without first thawing the
       fs will return -EBUSY.

       During this function, sb->s_writers.frozen goes through these values:

SB_UNFROZEN
       File system is normal, all writes progress as usual.

SB_FREEZE_WRITE
       The file system is in the process of being frozen. New writes should be blocked, though page faults are still allowed. We wait for all
       writes to complete and then proceed to the next stage.

SB_FREEZE_PAGEFAULT
       Freezing continues. Now also page faults are blocked but internal fs threads can still modify the filesystem (although they should not
       dirty new pages or inodes), writeback can run etc. After waiting for all running page faults we sync the filesystem which will clean all
       dirty pages and inodes (no new dirty pages or inodes can be created when sync is running).

SB_FREEZE_FS
       The file system is frozen. Now all internal sources of fs modification are blocked (e.g. XFS preallocation truncation on inode reclaim).
       This is usually implemented by blocking new transactions for filesystems that have them and need this additional guard. After all internal
       writers are finished we call ->freeze_fs to finish filesystem freezing. Then we transition to SB_FREEZE_COMPLETE state. This state is
       mostly auxiliary for filesystems to verify they do not modify frozen fs.

       sb->s_writers.frozen is protected by sb->s_umount.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014							   FREEZE_SUPER(9)