On Wed, 9 Jul 2008 21:49:58 +1000
Dave Chinner [off-list ref] wrote:

(e) none of the above.  The kernel compilation will appear to pause
until the filesystem is unfrozen. No other visible effect should
occur. It will get blocked in a write or filesystem transaction
because the fs is frozen.

Look at vfs_check_frozen() - any call to that will block if the
filesystem is frozen or being frozen. The generic hook is in
__generic_file_aio_write_nolock() and various other filesystems have
calls in their specific write paths (fuse, ntfs, ocfs2, xfs, xip) to
do this.

yeah and mmap doesn't happen

For all other modifications, filesystem specific methods of
blocking transactions are used. XFS uses vfs_check_frozen() in
xfs_trans_alloc(), ext3 (and probably ocfs2) do it via
their ->write_super_lockfs method calling journal_lock_updates(),
ext4 via jbd2_lock_updates() and so on....

and what if it's the process that you need to unfreeze the fs later?
Good luck.

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