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Subject: Re: [RFC] mmap(MAP_CONTIG)
To: Mike Kravetz <mike.kravetz@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
References: <21f1ec96-2822-1189-1c95-79a2bb491571@oracle.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
        Michal Nazarewicz <mina86@mina86.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, Guy Shattah <sguy@mellanox.com>,
        Christoph Lameter <cl@linux.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <b2dee13d-a19a-2b53-7317-7227749375d9@intel.com>
Date: Mon, 23 Oct 2017 15:10:05 -0700
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On 10/03/2017 04:56 PM, Mike Kravetz wrote:
> mmap(MAP_CONTIG) would have the following semantics:
> - The entire mapping (length size) would be backed by physically contiguous
>   pages.
> - If 'length' physically contiguous pages can not be allocated, then mmap
>   will fail.
> - MAP_CONTIG only works with MAP_ANONYMOUS mappings.
> - MAP_CONTIG will lock the associated pages in memory.  As such, the same
>   privileges and limits that apply to mlock will also apply to MAP_CONTIG.
> - A MAP_CONTIG mapping can not be expanded.

Do you also need to lock out the NUMA migration APIs somehow?  What
about KSM (or does it already ignore VM_LOCKED)?

> - At fork time, private MAP_CONTIG mappings will be converted to regular
>   (non-MAP_CONTIG) mapping in the child.  As such a COW fault in the child
>   will not require a contiguous allocation.
Maybe we should just define it as acting as if it had MADV_DONTFORK set
on it, and also that it doesn't allow MADV_DONTFORK to be called on it.