linux-kernel - Re: [PATCH] mm, memcg: sync allocation and memcg charge gfp flags for THP

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Message-ID: <5509A31C.3070108@suse.cz>
Date:	Wed, 18 Mar 2015 17:09:00 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, memcg: sync allocation and memcg charge gfp flags
 for THP

On 03/18/2015 04:59 PM, Michal Hocko wrote:
> On Wed 18-03-15 16:40:34, Vlastimil Babka wrote:
>> On 03/18/2015 04:02 PM, Michal Hocko wrote:
>>> On Wed 18-03-15 15:34:50, Vlastimil Babka wrote:
>>>> On 03/16/2015 03:08 PM, Michal Hocko wrote:
>>>>> @@ -1080,6 +1080,7 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
>>>>>   	unsigned long haddr;
>>>>>   	unsigned long mmun_start;	/* For mmu_notifiers */
>>>>>   	unsigned long mmun_end;		/* For mmu_notifiers */
>>>>> +	gfp_t huge_gfp = GFP_TRANSHUGE;	/* for allocation and charge */
>>>>
>>>> This value is actually never used. Is it here because the compiler emits a
>>>> spurious non-initialized value warning otherwise? It should be easy for it
>>>> to prove that setting new_page to something non-null implies initializing
>>>> huge_gfp (in the hunk below), and NULL new_page means it doesn't reach the
>>>> mem_cgroup_try_charge() call?
>>>
>>> No, I haven't tried to workaround the compiler. It just made the code
>>> more obvious to me. I can remove the initialization if you prefer, of
>>> course.
>>
>> Yeah IMHO it would be better to remove it, if possible. Leaving it has the
>> (albeit small) chance that future patch will again use the value in the code
>> before it's determined based on defrag setting.
>
> Wouldn't an uninitialized value be used in such a case?

Yeah, but then you should get a (correct) warning :)
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