Re: [PATCH 7/7] porting: document block device freeze and thaw changes

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Sep 27, 2023 at 03:21:20PM +0200, Christian Brauner wrote:
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  Documentation/filesystems/porting.rst | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
> index 4d05b9862451..fef97a2e6729 100644
> --- a/Documentation/filesystems/porting.rst
> +++ b/Documentation/filesystems/porting.rst
> @@ -1045,3 +1045,28 @@ filesystem type is now moved to a later point when the devices are closed:
>  As this is a VFS level change it has no practical consequences for filesystems
>  other than that all of them must use one of the provided kill_litter_super(),
>  kill_anon_super(), or kill_block_super() helpers.
> +
> +---
> +
> +**mandatory**
> +
> +Block device freezing and thawing have been moved to holder operations. As we
> +can now go straight from block devcie to superblock the get_active_super()

s/devcie/device/

> +and bd_fsfreeze_sb members in struct block_device are gone.
> +
> +The bd_fsfreeze_mutex is gone as well since we can rely on the bd_holder_lock
> +to protect against concurrent freeze and thaw.
> +
> +Before this change, get_active_super() would only be able to find the
> +superblock of the main block device, i.e., the one stored in sb->s_bdev. Block
> +device freezing now works for any block device owned by a given superblock, not
> +just the main block device.

You might want to document this new fs_holder_ops scheme:

"Filesystems opening a block device must pass the super_block object
and fs_holder_ops as the @holder and @hops parameters."

Though TBH I see a surprising amount of fs code that doesn't do this, so
perhaps it's not so mandatory?

--D

> +
> +When thawing we now grab an active reference so we can hold bd_holder_lock
> +across thaw without the risk of deadlocks (because the superblock goes away
> +which would require us to take bd_holder_lock). That allows us to get rid of
> +bd_fsfreeze_mutex. Currently we just reacquire s_umount after thaw_super() and
> +drop the active reference we took before. This someone could grab an active
> +reference before we dropped the last one. This shouldn't be an issue. If it
> +turns out to be one we can reshuffle the code to simply hold s_umount when
> +thaw_super() returns and drop the reference.
> 
> -- 
> 2.34.1
>