linux-kernel - Re: [PATCH V3] mm/thp: Allocate transparent hugepages on local node

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Date:	Tue, 20 Jan 2015 11:22:58 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3] mm/thp: Allocate transparent hugepages on local node

Vlastimil Babka <vbabka@...e.cz> writes:

> On 01/17/2015 01:02 AM, Andrew Morton wrote:
>> On Fri, 16 Jan 2015 12:56:36 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com> wrote:
>> 
>>> This make sure that we try to allocate hugepages from local node if
>>> allowed by mempolicy. If we can't, we fallback to small page allocation
>>> based on mempolicy. This is based on the observation that allocating pages
>>> on local node is more beneficial than allocating hugepages on remote node.
>> 
>> The changelog is a bit incomplete.  It doesn't describe the current
>> behaviour, nor what is wrong with it.  What are the before-and-after
>> effects of this change?
>> 
>> And what might be the user-visible effects?
>> 
>>> --- a/mm/mempolicy.c
>>> +++ b/mm/mempolicy.c
>>> @@ -2030,6 +2030,46 @@ retry_cpuset:
>>>  	return page;
>>>  }
>>>  
>>> +struct page *alloc_hugepage_vma(gfp_t gfp, struct vm_area_struct *vma,
>>> +				unsigned long addr, int order)
>> 
>> alloc_pages_vma() is nicely documented.  alloc_hugepage_vma() is not
>> documented at all.  This makes it a bit had for readers to work out the
>> difference!
>> 
>> Is it possible to scrunch them both into the same function?  Probably
>> too messy?
>
> Hm that could work, alloc_pages_vma already has an if (MPOL_INTERLEAVE) part, so
> just put the THP specialities into an "else if (huge_page)" part there?
>
> You could probably test for GFP_TRANSHUGE the same way as __alloc_pages_slowpath
> does. There might be false positives theoretically, but is there anything else
> that would use these flags and not be a THP?
>

is that check correct ? ie, 

if ((gfp & GFP_TRANSHUGE) == GFP_TRANSHUGE)

may not always indicate transparent hugepage if defrag = 0 . With defrag
cleared, we remove __GFP_WAIT from GFP_TRANSHUGE.

static inline gfp_t alloc_hugepage_gfpmask(int defrag, gfp_t extra_gfp)
{
	return (GFP_TRANSHUGE & ~(defrag ? 0 : __GFP_WAIT)) | extra_gfp;
}

-aneesh

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