Re: [PATCH 3/3] loop: Charge i/o to mem and blk cg

[Dan Schatzberg <schatzberg.dan@gmail.com>]

> This is how I understand it:
> 
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -996,6 +996,7 @@ static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd)
>         rb_insert_color(&worker->rb_node, &lo->worker_tree);
>  queue_work:
>         if (worker) {
> +               WARN_ON_ONCE(worker->blkcg_css != cmd->blkcg_css);

Yes, this is correct. Though the check here seems a bit obvious to me
- it must be correct because we assign worker above:

if (cur_worker->blkcg_css == cmd->blkcg_css) {
        worker = cur_worker;
        break;

or when we construct the worker:

worker->blkcg_css = cmd->blkcg_css;

I think this WARN_ON_ONCE check might be more interesting in
loop_process_work which invokes loop_handle_cmd and actually uses
cmd->blkcg_css. In any event, your understanding is correct here.

>                 /*
>                  * We need to remove from the idle list here while
>                  * holding the lock so that the idle timer doesn't
> @@ -2106,6 +2107,8 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
>         cmd->memcg_css = NULL;
>  #ifdef CONFIG_BLK_CGROUP
>         if (rq->bio && rq->bio->bi_blkg) {
> +               /* reference to blkcg_css will be held by loop_worker (outlives
> +                * cmd) or it is the eternal root css */

Yes, this is correct. Feel free to add my Acked-by to such a patch