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

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Date:	Wed, 21 Jan 2015 12:28:05 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	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

On 01/20/2015 10:08 AM, Vlastimil Babka wrote:
> On 01/20/2015 06:52 AM, Aneesh Kumar K.V wrote:
>> Vlastimil Babka <vbabka@...e.cz> writes:
>> 
>> 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.
> 
> Yep, that looks wrong. Sigh. I guess we can't spare an extra GFP flag to
> indicate TRANSHUGE?

I wanted to fix this in __alloc_pages_slowpath(), but actually there's no issue
(other than being quite subtle) - if defrag == 0 and thus we don't have
__GFP_WAIT, we reach "if (!wait) goto nopage;" and bail out before reaching the
checks for 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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> 
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