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

From: Vlastimil Babka <vbabka@suse.cz>
To: Michal Hocko <mhocko@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm, memcg: sync allocation and memcg charge gfp flags for THP
Date: Wed, 18 Mar 2015 16:40:34 +0100	[thread overview]
Message-ID: <55099C72.1080102@suse.cz> (raw)
In-Reply-To: <20150318150257.GL17241@dhcp22.suse.cz>

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.