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

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Message-ID: <alpine.DEB.1.10.0910010944480.26219@gentwo.org>
Date:	Thu, 1 Oct 2009 09:47:35 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
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 Thu, 1 Oct 2009, Hugh Dickins wrote:

> 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.

Look at how compound pages are handled in include/linux/page-flags.h

> 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.

Of course you need to be careful not to use two bits that can be used
indepedently.

> > 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.

We made the page flags stuff configurable. On 64 bit we use more flags, on
32 bit we compress the page flags a bit. Maybe do the same for vm_flags?


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