linux-kernel - Re: [patch 1/2] x86: track memtype for RAM in page struct

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Date:	Tue, 30 Sep 2008 14:14:18 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: Re: [patch 1/2] x86: track memtype for RAM in page struct - v3

On Tue, Sep 30, 2008 at 04:21:28AM -0700, Ingo Molnar wrote:
> 
> * Nick Piggin <nickpiggin@...oo.com.au> wrote:
> 
> > On Thursday 25 September 2008 01:53, Venki Pallipadi wrote:
> >
> > >  /*
> > > + * RED-PEN:  TODO: Add PageReserved() check as well here,
> > > + * once we add SetPageReserved() to all the drivers using
> > > + * set_memory_* or set_pages_*.
> > > + *
> > > + * This will help prevent accidentally freeing pages
> > > + * before setting the attribute back to WB.
> > > + */
> >
> > I'd rather we didn't add any more uses of PageReserved without a
> > really good reason.
> >
> > At this point in time (or at least last time I looked, a year or
> > two ago), it isn't a whole lot of work to remove PG_reserved
> > completely. If the waters get muddied up again, it could require
> > another significant rework to remove it in future...
> 
> agreed.

If a driver by mistake free's a RAM page before changing its memory attribute
back to WB, we want the generic -mm to catch it.

Today, free_pages_check() prevents freeing the page with PageReserved set. We
want to use this and make sure that either set_page_uc/wc() or the driver
calling these API's set the PageReserved bit. There are already some drivers
which do SetPageReserved() before changing the attribute.

I don't know the history behind PageReserved. But is there a recommended
way to achieve what we want? Either we need to use PageReserved bit or  add
some arch specific checks (in the x86 case, check arch_1) in free_pages_check().
Right?

thanks,
suresh
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