Re: [PATCH v6 4/5] writeback, cgroup: support switching multiple inodes at once

[Jan Kara]

On Wed 02-06-21 17:55:16, Roman Gushchin wrote:
> Currently only a single inode can be switched to another writeback
> structure at once. That means to switch an inode a separate
> inode_switch_wbs_context structure must be allocated, and a separate
> rcu callback and work must be scheduled.
> 
> It's fine for the existing ad-hoc switching, which is not happening
> that often, but sub-optimal for massive switching required in order to
> release a writeback structure. To prepare for it, let's add a support
> for switching multiple inodes at once.
> 
> Instead of containing a single inode pointer, inode_switch_wbs_context
> will contain a NULL-terminated array of inode
> pointers. inode_do_switch_wbs() will be called for each inode.
> 
> Signed-off-by: Roman Gushchin <guro@xxxxxx>

Two small comments below:

> @@ -473,10 +473,14 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
>  {
>  	struct inode_switch_wbs_context *isw =
>  		container_of(to_rcu_work(work), struct inode_switch_wbs_context, work);
> +	struct inode **inodep;
> +
> +	for (inodep = &isw->inodes[0]; *inodep; inodep++) {
                      ^^^^ why not just isw->inodes?

> +		inode_do_switch_wbs(*inodep, isw->new_wb);
> +		iput(*inodep);
> +	}

I was kind of hoping that we would save the repeated locking of
bdi->wb_switch_rwsem, old_wb->list_lock, and new_wb->list_lock for multiple
inodes. Maybe we can have 'old_wb' as part of isw as well and assert that
all inodes are still attached to the old_wb at this point to make this a
bit simpler. Or we can fetch old_wb from the first inode and then just
lock & assert using that one.

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