Re: [PATCH RFC v2 08/18] fs: add dedicated block device open helpers for filesystems

[Jan Kara]

On Mon 22-06-26 18:28:50, Jan Kara wrote:
> On Tue 16-06-26 16:08:24, Christian Brauner wrote:
> > Add fs_bdev_file_open_by_{dev,path}() and fs_bdev_file_release(). They
> > open the device with fs_holder_ops and register a claim in the
> > device-to-superblock table. Claims on the same (device, superblock)
> > pair share one entry, so when a filesystem claims a device it already
> > uses (xfs with its log on the data device), no second entry is added
> > and each superblock will be acted on once.
> > 
> > The holder argument remains purely the block layer's exclusivity token:
> > a superblock, or a file_system_type for a device shared by several
> > superblocks of that type. The shared case only becomes usable once the
> > fs_holder_ops callbacks resolve superblocks through the table instead
> > of bdev->bd_holder.
> > 
> > Convert the main device, setup_bdev_super() and kill_block_super(),
> > over: the open finds the entry registered by sget_fc() and claims it
> > again. cramfs and romfs bypass kill_block_super() so they can handle
> > MTD mounts and release the main device with a plain bdev_fput(), which
> > would leave the claim behind: the (dev, sb) entry would never be
> > unregistered and the passive reference it holds would keep the
> > superblock alive forever. Convert their release paths in the same
> > step.
> > 
> > The frozen-device check stays in setup_bdev_super() for the primary
> > device and is added to fs_bdev_register() for new claims, i.e. every
> > additional device a filesystem opens through the helpers. Only a
> > (device, superblock) pair the superblock claimed earlier may be
> > reopened while frozen (xfs with its log on the data device): the freeze
> > already covers that superblock through the existing claim, so nothing
> > escapes it. Without the setup_bdev_super() check a device frozen before
> > the mount even started (dm lock_fs, loop) could be mounted and written
> > to (journal replay) under an active freeze, because the primary open
> > reuses the entry registered by sget_fc() and never takes the new-claim
> > path.
> > 
> > Both checks read bd_fsfreeze_count only after the entry is published
> > (by sget_fc() for the primary, by fs_bdev_register() for new claims)
> > and pair with bdev_freeze() incrementing the count before walking the
> > table: either the mount sees the elevated freeze count and fails with
> > EBUSY, or the freeze finds the published entry and converges once
> > SB_BORN is set.
> > 
> > Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>

...

> > +/**
> > + * fs_bdev_file_release - release a block device claimed for a superblock
> > + * @bdev_file: file returned by fs_bdev_file_open_by_{dev,path}()
> > + * @sb: superblock the device was claimed for
> > + *
> > + * Drop one claim on the {dev, @sb} entry; the last claim unregisters it (a
> > + * pinning cursor defers the actual unlink).  Then close the block device.
> > + */
> > +void fs_bdev_file_release(struct file *bdev_file, struct super_block *sb)
> > +{
> > +	dev_t dev = file_bdev(bdev_file)->bd_dev;
> > +	struct super_dev *sb_dev;
> > +
> > +	rcu_read_lock();
> > +	sb_dev = super_dev_lookup(dev, sb);
> > +	rcu_read_unlock();
> > +	super_dev_put(sb_dev);
> > +	bdev_fput(bdev_file);
> > +}
> > +EXPORT_SYMBOL_GPL(fs_bdev_file_release);
> 
> Why don't you use sb->s_super_dev in this function?

Nevermind, I've realized sb can hold multiple bdevs so this is really
needed.

I'd still prefer the __free handling in fs_bdev_register() sorted out but
regardless feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR