Re: Freezing (was: Re: fstests generic/441 -- occasional bcachefs failure)

[Kent Overstreet <kent.overstreet@linux.dev>]

On Fri, Feb 03, 2023 at 11:51:12AM +1100, Dave Chinner wrote:
> What do you need to know? The vast majority of the freeze
> infrastructure is generic and the filesystem doesn't need to do
> anything special. The only thing it needs to implement is
> ->freeze_fs to essentially quiesce the filesystem - by this stage
> all the data has been written back and all user-driven operations
> have either been stalled or drained at either the VFS or transaction
> reservation points.
> 
> This requires the filesystem transaction start point to call
> sb_start_intwrite() in a location the transaction start can block
> safely forever, and to call sb_end_intwrite() when the transaction
> is complete and being torn down. 
> 
> [Note that bcachefs might also require
> sb_{start/end}_{write/pagefault} calls in paths that it has custom
> handlers for and to protect against ioctl operations triggering
> modifications during freezes.]
> 
> This allows freeze_super() to set a barrier to prevent new
> transactions from starting, and to wait on transactions in flight to
> drain. Once all transactions have drained, it will then call
> ->freeze_fs if it is defined so the filesystem can flush it's
> journal and dirty in-memory metadata so that it becomes consistent
> on disk without requiring journal recovery to be run.
> 
> This basically means that once ->fs_freeze completes, the filesystem
> should be in the same state on-disk as if it were unmounted cleanly,
> and the fs will not issue any more IO until the filesystem is
> thawed. Thaw will call ->unfreeze_fs if defined before unblocking
> tasks so that the filesystem can restart things that may be needed
> for normal operation that were stopped during the freeze.
> 
> It's not all that complex anymore - a few hooks to enable
> modification barriers to be placed and running the
> writeback part of unmount in ->freeze_fs is the main component
> of the work that needs to be done....

Thanks, that is a lot simpler than I was thinking - I guess I was
thinking about task freezing for suspend, that definitely had some
tricky bits. Sounds like treating it as if we were remounting read-only
is all we need to do.

I'll get on this when I get done with backpointers, unless someone else
is willing to work on it :)