Re: [PATCH v9 2/3] memcg,writeback: flush foreign bdev mappings separately from owner wbs

[Christoph Hellwig <hch@infradead.org>]

> +/*
> + * No extra memcg reference is taken: this work is embedded in the memcg,
> + * and mem_cgroup_css_free() waits for it to finish before freeing the memcg.
> + */
> +struct memcg_bdev_frn {

No reason not to spell out foreign here..

> +		/* Reserve the slot: it may have become busy after the inflight check. */

Overly long line.

> +	struct address_space *mapping = folio_mapping(folio);
> +	struct inode *inode = mapping->host;
> +
> +	if (memcg_bdev_frn_wq && sb_is_blkdev_sb(inode->i_sb)) {

Why do we need a NULL check for the workqueue here?

>  	memcg_wq = alloc_workqueue("memcg", WQ_PERCPU, 0);
>  	WARN_ON(!memcg_wq);
>  
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +	memcg_bdev_frn_wq = alloc_workqueue("memcg_bdev_frn_flusher",
> +					    WQ_UNBOUND, 0);
> +	WARN_ON(!memcg_bdev_frn_wq);
> +#endif

Instead please fail the boot.  If we can't alloc a workqueue at
boot time, we're toast anyway.