linux-kernel - Re: [PATCH 0/1] pagemap clear_refs: modify to specify anon or mapped vma clearing

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Date:	Thu, 23 Jul 2009 22:46:17 -0700
From:	"Moussa A. Ba" <moussa.a.ba@...il.com>
To:	Matt Mackall <mpm@...enic.com>
CC:	linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>,
	akpm@...ux-foundation.org, adobriyan@...il.com, mel@....ul.ie,
	yinghan@...gle.com, npiggin@...e.de, jaredeh@...il.com
Subject: Re: [PATCH 0/1] pagemap clear_refs: modify to specify anon or mapped
 vma clearing

Matt Mackall wrote:
> On Thu, 2009-07-23 at 20:56 -0700, Moussa A. Ba wrote:
>> This patch makes the clear_refs proc interface a bit more versatile. It 
>> adds support for clearing either anonymous, file mapped pages or both.
>>
>> echo 1 > /proc/pid/clear_refs clears ANON pages
>> echo 2 > /proc/pid/clear_refs clears file mapped pages
>> echo 3 > /proc/pid/clear_refs clears all pages
>>
>> Selectively clearing pages has a measurable impact on performance as it 
>> decreases the number of page walks.  We have been using this interface 
>> and this adds flexibility to the user space application implementing the 
>> reference clearing.
> 
> [adding David, the original instigator of clear_refs)
> 
> First, don't send 0 of 1 messages, especially for a single patch. It
> makes Andrew curse and makes him more likely to consider other
> employment possibilities. Instead incorporate highly useful info like
> the above into your patch instead, preferably as docs and commit
> descriptions.
> 
> Second, depending on how we've documented the behavior of this file,
> this looks like it might break existing tools that are currently writing
> '1' but expecting the behavior of '3'?
> 

Thank you for the kind advice, I was afraid of that...  In regards to 
the behavior of 1 vs 3, this had indeed occurred to us, and it was a 
tossup.  I was hoping to get feedback first before deciding which way to 
go.  However, it does seem like a better idea to go with the default 
behavior when writing '1' and the special behaviors for 2 and 3.


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