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

[Balbir Singh]

Peter Zijlstra wrote:
>> From: Balbir Singh <balbir at linux.vnet.ibm.com>
> 
>>  void page_assign_page_cgroup(struct page *page, struct page_cgroup *pc)
>>  {
>> -	page->page_cgroup = (unsigned long)pc;
>> +	int locked;
>> +
>> +	/*
>> +	 * While resetting the page_cgroup we might not hold the
>> +	 * page_cgroup lock. free_hot_cold_page() is an example
>> +	 * of such a scenario
>> +	 */
>> +	if (pc)
>> +		VM_BUG_ON(!page_cgroup_locked(page));
>> +	locked = (page->page_cgroup & PAGE_CGROUP_LOCK);
>> +	page->page_cgroup = ((unsigned long)pc | locked);
>>  }
> 
> This looks a bit odd, why not write:
> 
>  locked = page_cgroup_locked(page);
>  if (pc)
>    VM_BUG_ON(!locked)
> 

Sure, we could write it this way or

VM_BUG_ON(pc && !locked)

>> +/*
>> + * 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.

>> +	pc->mem_cgroup = mem;
>> +	pc->page = page;
>> +	page_assign_page_cgroup(page, pc);
>> +
>> +done:
>> +	unlock_page_cgroup(page);
>> +	return 0;
>> +free_pc:
>> +	kfree(pc);
>> +	return -ENOMEM;
>> +err:
>> +	unlock_page_cgroup(page);
>> +	return -ENOMEM;
>> +}
> 
> 
> 
>> @@ -2161,6 +2184,9 @@ static int do_anonymous_page(struct mm_s
>>  	if (!page)
>>  		goto oom;
>>  
>> +		if (mem_cgroup_charge(page, mm))
>> +			goto oom_free_page;
>> +
>>  	entry = mk_pte(page, vma->vm_page_prot);
>>  	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>>  
> 
> whitespace damage
> 

Yes, it's already been fixed in Andrew's tree. Paul could you
please pull in the those fixes as well? They are not in a -mm
tree, but you can find them on mm-commits.



-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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