linux-kernel - Re: [RFC PATCH 0/3] Add mmap(MAP

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Message-Id: <f1737666-b65e-38e2-94af-129e66031503@linux.vnet.ibm.com>
Date:   Thu, 12 Oct 2017 16:06:13 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Cc:     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>,
        Christoph Lameter <cl@...ux.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Laura Abbott <labbott@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC PATCH 0/3] Add mmap(MAP_CONTIG) support

On 10/12/2017 07:16 AM, Mike Kravetz wrote:
> The following is a 'possible' way to add such functionality.  I just
> did what was easy and pre-allocated contiguous pages which are used
> to populate the mapping.  I did not use any of the higher order
> allocators such as alloc_contig_range.  Therefore, it is limited to

Just tried with a small prototype with an implementation similar to that
of alloc_gigantic_page() where we scan the zones (applicable zonelist)
for contiguous valid PFN range and try allocating with alloc_contig_range.
Will share it soon.

> allocations of MAX_ORDER size.  Also, the allocations should probably

Just did a quick test and it worked till 1UL << (MAX_ORDER - 1) numbers
of pages on a POWER system with the current RFC patches. As the pages
are allocated during VMA creation time, comparison to normal page fault
speed while accessing the buffer wont be fair.

> be done outside mmap_sem but that was the easiest place to do it in
> this quick and easy POC.

Why it should be done outside the mmap_sem, because it can take some
time ? But then VMA can just go away while we are allocating the big
chunks of pages (if we dont hold mmap_sem).