linux-kernel - Re: [RFC] mmap(MAP

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Date:   Wed, 4 Oct 2017 11:05:33 -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>
Subject: Re: [RFC] mmap(MAP_CONTIG)

On Wed, 4 Oct 2017, Anshuman Khandual wrote:

> > - Using 'pre-allocated' pages in the fault paths may be intrusive.
>
> But we have already faulted in all of them for the mapping and they
> are also locked. Hence there should not be any page faults any more
> for the VMA. Am I missing something here ?

The PTEs may be torn down and have to reestablished through a page faults.
Page faults would not allocate memory.

> I am still wondering why wait till fault time not pre fault all of them
> and populate the page tables.

They are populated but some processes (swap and migration) may tear them
down.