Re: [patch 1/2] x86: track memtype for RAM in page struct - v3

[Ingo Molnar <mingo@elte.hu>]

* Suresh Siddha <suresh.b.siddha@intel.com> wrote:

> On Tue, Sep 30, 2008 at 04:21:28AM -0700, Ingo Molnar wrote:
> > 
> > * Nick Piggin <nickpiggin@yahoo.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.

no, the generic -mm does not 'catch' PageReserved, it simply _ignores_ 
it. So i agree about having a debug check there, it's just that what you 
propose does not achieve that.

PageReserved is a legacy thing that should not be used in new code.  
Using it will only hide bugs and adds quirkiness to the PAT code.

> 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?

the best way i think would be to add arch_1 to PAGE_FLAGS_CHECK_AT_FREE. 
That way the mm becomes very noisy if this is ever freed.

Nick, Andrew, any preferences?

	Ingo