linux-kernel - Re: Where in the code mmap + MAP_ANONYMOUS clears returned pages?

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Date:	Tue, 15 Jul 2014 21:14:40 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Alexei Fedotov <alexei.fedotov@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Where in the code mmap + MAP_ANONYMOUS clears returned pages?

On Tue 15-07-14 21:55:03, Alexei Fedotov wrote:
> Guys,
> 
> I look into mmap.c code [1] and try to understand where anonymous
> memory mapping (mmap + MAP_ANONYMOUS) sets memory to zero. My best
> guess is I should dig that somewhere in mmap_region.
> 
> Could you please help me locating the code which makes returned memory
> pages clean? Thank you in advance.

mmap doesn't allocate pages which back the mapping directly unless it is
told to by MAP_POPULATE. So the clean pages are either allocated during
the page fault path when the memory is accessed (see do_anonymous_page
for private 4k pages fault, do_huge_pmd_anonymous_page for THP page and
MAP_SHARED is left as an excercise for the reader ;)) or in mmap path
directly when MAP_POPULATE is specified (see vm_mmap_pgoff which calls
mm_populate)

> 
> [1] http://code.ohloh.net/file?fid=RPa2fuiQ5j_IwR1nymptYBBC3X4&cid=w6qrXNDsMqk
> 
> --
> With best regards / с наилучшими пожеланиями,
> Alexei Fedotov / Алексей Федотов,
> http://dataved.ru/
> +7 916 562 8095
> 
> [1] Start using Apache Openmeetings today, http://openmeetings.apache.org/
> [2] Join Alexei Fedotov @linkedin, http://ru.linkedin.com/in/dataved/
> [3] Join Alexei Fedotov @facebook, http://www.facebook.com/openmeetings
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-- 
Michal Hocko
SUSE Labs
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