[Devel] Re: [PATCH 23/29] memory controller memory accounting v7

[Peter Zijlstra]

On Thu, 2007-09-13 at 15:19 +0530, Balbir Singh wrote:

> VM_BUG_ON(pc && !locked)

Even better :-)

> >> +/*
> >> + * Charge the memory controller for page usage.
> >> + * Return
> >> + * 0 if the charge was successful
> >> + * < 0 if the cgroup is over its limit
> >> + */
> >> +int mem_cgroup_charge(struct page *page, struct mm_struct *mm)
> >> +{
> >> +	struct mem_cgroup *mem;
> >> +	struct page_cgroup *pc, *race_pc;
> >> +
> >> +	/*
> >> +	 * Should page_cgroup's go to their own slab?
> >> +	 * One could optimize the performance of the charging routine
> >> +	 * by saving a bit in the page_flags and using it as a lock
> >> +	 * to see if the cgroup page already has a page_cgroup associated
> >> +	 * with it
> >> +	 */
> >> +	lock_page_cgroup(page);
> >> +	pc = page_get_page_cgroup(page);
> >> +	/*
> >> +	 * The page_cgroup exists and the page has already been accounted
> >> +	 */
> >> +	if (pc) {
> >> +		atomic_inc(&pc->ref_cnt);
> >> +		goto done;
> >> +	}
> >> +
> >> +	unlock_page_cgroup(page);
> >> +
> >> +	pc = kzalloc(sizeof(struct page_cgroup), GFP_KERNEL);
> >> +	if (pc == NULL)
> >> +		goto err;
> >> +
> >> +	rcu_read_lock();
> >> +	/*
> >> +	 * We always charge the cgroup the mm_struct belongs to
> >> +	 * the mm_struct's mem_cgroup changes on task migration if the
> >> +	 * thread group leader migrates. It's possible that mm is not
> >> +	 * set, if so charge the init_mm (happens for pagecache usage).
> >> +	 */
> >> +	if (!mm)
> >> +		mm = &init_mm;
> >> +
> >> +	mem = rcu_dereference(mm->mem_cgroup);
> >> +	/*
> >> +	 * For every charge from the cgroup, increment reference
> >> +	 * count
> >> +	 */
> >> +	css_get(&mem->css);
> >> +	rcu_read_unlock();
> >> +
> >> +	/*
> >> +	 * If we created the page_cgroup, we should free it on exceeding
> >> +	 * the cgroup limit.
> >> +	 */
> >> +	if (res_counter_charge(&mem->res, 1)) {
> >> +		css_put(&mem->css);
> >> +		goto free_pc;
> >> +	}
> >> +
> >> +	lock_page_cgroup(page);
> >> +	/*
> >> +	 * Check if somebody else beat us to allocating the page_cgroup
> >> +	 */
> >> +	race_pc = page_get_page_cgroup(page);
> >> +	if (race_pc) {
> >> +		kfree(pc);
> >> +		pc = race_pc;
> >> +		atomic_inc(&pc->ref_cnt);
> > 
> > This inc
> > 
> >> +		res_counter_uncharge(&mem->res, 1);
> >> +		css_put(&mem->css);
> >> +		goto done;
> >> +	}
> >> +
> >> +	atomic_set(&pc->ref_cnt, 1);
> > 
> > combined with this set make me wonder...
> > 
> 
> I am not sure I understand this comment.

Is that inc needed? the pc is already associated with the page and
should thus already have a reference, so this inc would do 1->2, but we
then set it to 1 again. seems like a superfluous operation.
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