Re: [PATCH] buffer: Associate the meta bio with blkg from buffer page

[Tejun Heo <tj@kernel.org>]

Hello, Haifeng.

On Wed, Aug 28, 2024 at 11:32:24AM +0800, Haifeng Xu wrote:
...
> The filesystem is ext4(ordered). The meta data can be written out by
> writeback, but if there are too many dirty pages, we had to do
> checkpoint to write out the meta data in current thread context.
> 
> In this case, the blkg of thread1 has set io.max, so the j_checkpoint_mutex
> can't be released and many threads must wait for it. However, the blkg from
> buffer page didn' set any io policy. Therefore, for the meta buffer head,
> we can associate the bio with blkg from the buffer page instead of current
> thread context.
> 
> Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
> ---
>  fs/buffer.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index e55ad471c530..a7889f258d0d 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -2819,6 +2819,17 @@ static void submit_bh_wbc(blk_opf_t opf, struct buffer_head *bh,
>  	if (wbc) {
>  		wbc_init_bio(wbc, bio);
>  		wbc_account_cgroup_owner(wbc, bh->b_page, bh->b_size);
> +	} else if (buffer_meta(bh)) {
> +		struct folio *folio;
> +		struct cgroup_subsys_state *memcg_css, *blkcg_css;
> +
> +		folio = page_folio(bh->b_page);
> +		memcg_css = mem_cgroup_css_from_folio(folio);
> +		if (cgroup_subsys_on_dfl(memory_cgrp_subsys) &&
> +		    cgroup_subsys_on_dfl(io_cgrp_subsys)) {
> +			blkcg_css = cgroup_e_css(memcg_css->cgroup, &io_cgrp_subsys);
> +			bio_associate_blkg_from_css(bio, blkcg_css);

I think the right way to do it is marking the bio with REQ_META and
implement forced charging in blk-throtl similar to blk-iocost.

Thanks.

-- 
tejun