On Wed, Sep 27, 2023 at 03:21:20PM +0200, Christian Brauner wrote: > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> > --- > 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 >
