linux-kernel - Re: [PATCH] mm: page_alloc: Skip over regions of invalid pfns where possible

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Message-Id: <20170106144348.f7d207baa7b3190a95aaeb2e@linux-foundation.org>
Date:   Fri, 6 Jan 2017 14:43:48 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Paul Burton <paul.burton@...tec.com>
Cc:     <linux-mm@...ck.org>, James Hartley <james.hartley@...tec.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: Skip over regions of invalid pfns where
 possible

On Fri, 25 Nov 2016 18:55:18 +0000 Paul Burton <paul.burton@...tec.com> wrote:

> When using a sparse memory model memmap_init_zone() when invoked with
> the MEMMAP_EARLY context will skip over pages which aren't valid - ie.
> which aren't in a populated region of the sparse memory map. However if
> the memory map is extremely sparse then it can spend a long time
> linearly checking each PFN in a large non-populated region of the memory
> map & skipping it in turn.
> 
> When CONFIG_HAVE_MEMBLOCK_NODE_MAP is enabled, we have sufficient
> information to quickly discover the next valid PFN given an invalid one
> by searching through the list of memory regions & skipping forwards to
> the first PFN covered by the memory region to the right of the
> non-populated region. Implement this in order to speed up
> memmap_init_zone() for systems with extremely sparse memory maps.

Could we have a changelog which includes some timing measurements? 
That permits others to understand the value of this patch.