Re: [PATCH 05/13] swap,blkcg: issue swap io with the appropriate context

[Tejun Heo]

On Tue, May 29, 2018 at 05:17:16PM -0400, Josef Bacik wrote:
> From: Tejun Heo <tj@xxxxxxxxxx>
> 
> For backcharging we need to know who the page belongs to when swapping
> it out.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> ---
>  mm/page_io.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/page_io.c b/mm/page_io.c
> index a552cb37e220..61e1268e5dbc 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -339,6 +339,16 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
>  		goto out;
>  	}
>  	bio->bi_opf = REQ_OP_WRITE | REQ_SWAP | wbc_to_write_flags(wbc);
> +#if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
> +	if (page->mem_cgroup) {
> +		struct cgroup_subsys_state *blkcg_css;
> +
> +		blkcg_css = cgroup_get_e_css(page->mem_cgroup->css.cgroup,
> +					     &io_cgrp_subsys);
> +		bio_associate_blkcg(bio, blkcg_css);
> +		css_put(blkcg_css);
> +	}
> +#endif

So, this ignores the cases where bdev_write_page() is the one which
does the writes.  If my reading is correct, only brd, zram, btt and
pmem implement bdev_ops->rw_page() and take bdev_write_page() path, so
it shouldn't be a problem in majority of cases.

I don't think we need to address ->rw_page() case right now but it
might be a good idea to add a comment explaining the ommission.

Thanks.

-- 
tejun