freeze_super — lock the filesystem and force it into a consistent state
int freeze_super ( | struct super_block * sb); |
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:
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.
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).
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.