linux-kernel - Re: [RFC][PATCH] mm: madvise: MADV

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Date:	Tue, 2 Jul 2013 14:06:56 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

On Mon, Jul 01, 2013 at 09:43:29PM -0700, Dave Hansen wrote:
> On 07/01/2013 07:37 PM, Zheng Liu wrote:
> > FWIW, it would be great if we can let MAP_POPULATE flag support shared
> > mappings because in our product system there has a lot of applications
> > that uses mmap(2) and then pre-faults this mapping.  Currently these
> > applications need to pre-fault the mapping manually.
> 
> Are you sure it doesn't?  From a cursory look at the code, it looked to
> me like it would populate anonymous and file-backed, but I didn't
> double-check experimentally.

Thanks for pointing it out. I write a program to test this issue, and it
seems to me that it can populate a shared mapping.  But in manpage it
describes as below:

MAP_POPULATE (since Linux 2.5.46)
    Populate (prefault) page tables for a mapping.  For a file mapping,
    this causes read-ahead on the file.  Later accesses to the mapping
    will not be blocked by page faults.  MAP_POPULATE is only supported
    for private mappings since Linux 2.6.23.

This page is part of release 3.24 of the Linux man-pages project.  I am
not sure whether it has been updated or not.

Regards,
                                                - Zheng
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