Re: [PATCH 7/12] memcg add function to move account

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

> @@ -444,9 +445,14 @@ void mem_cgroup_move_lists(struct page *
>  
>  	pc = page_get_page_cgroup(page);
>  	if (pc) {
> +		mem = pc->mem_cgroup;
>  		mz = page_cgroup_zoneinfo(pc);
>  		spin_lock_irqsave(&mz->lru_lock, flags);
> -		__mem_cgroup_move_lists(pc, lru);
> +		/*
> +		 * check against the race with move_account.
> +		 */
> +		if (likely(mem == pc->mem_cgroup))
> +			__mem_cgroup_move_lists(pc, lru);

(snip)

> @@ -754,16 +824,24 @@ __mem_cgroup_uncharge_common(struct page
>  	if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
>  	    && ((PageCgroupCache(pc) || page_mapped(page))))
>  		goto unlock;
> -
> +retry:
> +	mem = pc->mem_cgroup;
>  	mz = page_cgroup_zoneinfo(pc);
>  	spin_lock_irqsave(&mz->lru_lock, flags);
> +	if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED &&
> +	    unlikely(mem != pc->mem_cgroup)) {
> +		/* MAPPED account can be done without lock_page().
> +		   Check race with mem_cgroup_move_account() */
> +		spin_unlock_irqrestore(&mz->lru_lock, flags);
> +		goto retry;
> +	}

I'm sorry, but I've not been convinced yet why these checks are needed here.
(Those checks are removed by [9/12] anyway.)

IIUC, pc->mem_cgroup is moved to another group only by mem_cgroup_move_account
under lock_page_cgroup( and mz->lru_lock).
And those two above(mem_cgroup_move_lists and __mem_cgroup_uncharge_common) sets
mem = pc->mem_cgroup under lock_page_cgroup, so I don't think those checks
(mem != pc->mem_cgroup) is needed.


Thanks,
Daisuke Nishimura.

> +/**
> + * mem_cgroup_move_account - move account of the page
> + * @page ... the target page of being moved.
> + * @pc   ... page_cgroup of the page.
> + * @from ... mem_cgroup which the page is moved from.
> + * @to   ... mem_cgroup which the page is moved to.
> + *
> + * The caller must confirm following.
> + * 1. disable irq.
> + * 2. lru_lock of old mem_cgroup should be held.
> + * 3. pc is guaranteed to be valid and on mem_cgroup's LRU.
> + *
> + * Because we cannot call try_to_free_page() here, the caller must guarantee
> + * this moving of charge never fails. (if charge fails, this call fails.)
> + * Currently this is called only against root cgroup.
> + * which has no limitation of resource.
> + * Returns 0 at success, returns 1 at failure.
> + */
> +int mem_cgroup_move_account(struct page *page, struct page_cgroup *pc,
> +	struct mem_cgroup *from, struct mem_cgroup *to)
> +{
> +	struct mem_cgroup_per_zone *from_mz, *to_mz;
> +	int nid, zid;
> +	int ret = 1;
> +
> +	VM_BUG_ON(!irqs_disabled());
> +
> +	nid = page_to_nid(page);
> +	zid = page_zonenum(page);
> +	from_mz =  mem_cgroup_zoneinfo(from, nid, zid);
> +	to_mz =  mem_cgroup_zoneinfo(to, nid, zid);
> +
> +	if (res_counter_charge(&to->res, PAGE_SIZE)) {
> +		/* Now, we assume no_limit...no failure here. */
> +		return ret;
> +	}
> +	if (!try_lock_page_cgroup(page)) {
> +		res_counter_uncharge(&to->res, PAGE_SIZE);
> +		return ret;
> +	}
> +
> +	if (page_get_page_cgroup(page) != pc) {
> +		res_counter_uncharge(&to->res, PAGE_SIZE);
> +		goto out;
> +	}
> +
> +	if (spin_trylock(&to_mz->lru_lock)) {
> +		__mem_cgroup_remove_list(from_mz, pc);
> +		css_put(&from->css);
> +		res_counter_uncharge(&from->res, PAGE_SIZE);
> +		pc->mem_cgroup = to;
> +		css_get(&to->css);
> +		__mem_cgroup_add_list(to_mz, pc);
> +		ret = 0;
> +		spin_unlock(&to_mz->lru_lock);
> +	} else {
> +		res_counter_uncharge(&to->res, PAGE_SIZE);
> +	}
> +out:
> +	unlock_page_cgroup(page);
> +
> +	return ret;
> +}
> +
>  /*
>   * Charge the memory controller for page usage.
>   * Return
>  	__mem_cgroup_remove_list(mz, pc);
>  	spin_unlock_irqrestore(&mz->lru_lock, flags);
>  
>  	page_assign_page_cgroup(page, NULL);
>  	unlock_page_cgroup(page);
>  
> -	mem = pc->mem_cgroup;
> +
>  	res_counter_uncharge(&mem->res, PAGE_SIZE);
>  	css_put(&mem->css);
>  
> 

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