linux-kernel - Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP

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Date:   Fri, 13 Oct 2017 10:14:16 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Guy Shattah <sguy@...lanox.com>,
        Laura Abbott <labbott@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC PATCH 3/3] mm/map_contig: Add mmap(MAP_CONTIG) support

On Thu, 12 Oct 2017, Anshuman Khandual wrote:

> > +static long __alloc_vma_contig_range(struct vm_area_struct *vma)
> > +{
> > +	gfp_t gfp = GFP_HIGHUSER | __GFP_ZERO;
>
> Would it be GFP_HIGHUSER_MOVABLE instead ? Why __GFP_ZERO ? If its
> coming from Buddy, every thing should have already been zeroed out
> in there. Am I missing something ?

Contiguous pages cannot and should not be moved. They will no longer be
contiguous then. Also the page migration code cannot handle this case.