linux-kernel - Re: No more bits in vm_area_struct's vm

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Message-ID: <Pine.LNX.4.64.0910011134240.10818@sister.anvils>
Date:	Thu, 1 Oct 2009 11:54:34 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Nigel Cunningham <ncunningham@...a.org.au>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: No more bits in vm_area_struct's vm_flags.

On Tue, 29 Sep 2009, Christoph Lameter wrote:
> 
> Another concern that has not been discussed is the increased cache
> footprint due to a slightly enlarged vm data working set (there is also a
> corresponding icache issue since additional accesses are needed).

Using unsigned long long vm_flags makes no difference to cache footprint
on 64-bit systems, being a no-op there; and I think these days, though
we sure like our 32-bit systems to run well, we're not so anxious about
saving every last cycle on them.

> 
> Could we stick with the current size and do combinations of flags like we
> do with page flags?

Are we doing that?  If you have some example like, when PG_slab is set
then PG_owner_priv_1 means such-and-such, but if not not: okay, I'm
fine with that.

But if you're saying something like, if PG_reclaim is set at the same
time as PG_buddy, then they mean the page is not a buddy or under
reclaim, but brokenbacked: then I'm a bit (or even 32 bits) worried.

> VM_HUGETLB cannot grow up and down f.e. and there are
> certainly lots of other impossible combinations that can be used to put
> more information into the flags.

Where it makes sense, where it's understandable, okay: there may be a
few which could naturally use combinations.  But in general, no, I
think we'd be asking for endless maintenance trouble if we change the
meaning of some flags according to other flags.

Hugh
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