linux-kernel - Re: [PATCH mm] limit the mm->map_count not greater than sysctl_max_map

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Date:	Wed, 18 Apr 2012 21:40:35 -0400
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH mm] limit the mm->map_count not greater than
 sysctl_max_map_count

On Wed, 2012-04-18 at 21:28 +0800, Cong Wang wrote:
> On 04/18/2012 05:27 PM, Li Zhong wrote:
> > When reading the mmap codes, I found the checking of mm->map_count
> > against sysctl_max_map_count is not consistent. At some places, ">" is
> > used; at some other places, ">=" is used.
> >
> > This patch changes ">" to">=", so they are consistent, and makes sure
> > the value is not greater (one more) than sysctl_max_map_count.
> >
> 
> Well, according to Documentation/sysctl/vm.txt,
> 
> max_map_count:
> 
> This file contains the maximum number of memory map areas a process
> may have. [...]
> 
> I think ->map_count == sysctl_max_map_count should be allowed, so using 
> '>' is correct.
> 
Yes, I agree that ->map_count == sysctl_max_map_count should be allowed.
However, with '>' used. The ->map_count could be sysctl_max_map_count+1.
It could be seen with a simple program doing continuously mmaping of a
file. 

( Still it is possible, as stated in the comments of do_munmap code, if
the VMA is going to be divided into two, the map_count could temporarily
be sysctl_max_map_count+1, after the original vma split into two, and
before one of the two vmas removed. ) 


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