Messages in this thread | | | Date | Wed, 1 Oct 2008 11:29:06 +0200 | From | Ingo Molnar <> | Subject | Re: [patch 1/2] x86: track memtype for RAM in page struct - v3 |
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* 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
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