linux-kernel - Re: [PATCH 3/8] sparc64: Eliminate PTE table memory wastage.

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Message-Id: <20121004.142003.851528112593506369.davem@davemloft.net>
Date:	Thu, 04 Oct 2012 14:20:03 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	aneesh.kumar@...ux.vnet.ibm.com
Cc:	linux-mm@...ck.org, sparclinux@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	akpm@...ux-foundation.org, aarcange@...hat.com, hannes@...xchg.org
Subject: Re: [PATCH 3/8] sparc64: Eliminate PTE table memory wastage.

From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Date: Thu, 04 Oct 2012 22:00:48 +0530

> David Miller <davem@...emloft.net> writes:
> 
>> We've split up the PTE tables so that they take up half a page instead
>> of a full page.  This is in order to facilitate transparent huge page
>> support, which works much better if our PMDs cover 4MB instead of 8MB.
>>
>> What we do is have a one-behind cache for PTE table allocations in the
>> mm struct.
>>
>> This logic triggers only on allocations.  For example, we don't try to
>> keep track of free'd up page table blocks in the style that the s390
>> port does.
> 
> I am also implementing a similar change for powerpc. We have a 64K page
> size, and want to make sure PMD cover 16MB, which is the huge page size
> supported by the hardware. I was looking at using the s390 logic,
> considering we have 16 PMDs mapping to same PTE page. Should we look at
> generalizing the case so that other architectures can start using the
> same code ?

I think until we have multiple cases we won't know what's common or not.

Each arch has different need.  I need to split the page into two pieces
so my code is simpler, and juse uses page counting to manage alloc/free.

Whereas s390 uses an bitmask to manage page state, and also reclaims
pgtable pages into a per-mm list on free.  I decided not to do that
and to just let the page allocator do the work.

So I don't think it's appropriate to think about commonization at this
time, as even the only two cases existing are very non-common :-)
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