linux-kernel - Re: [PATCH 33/36] autonuma: powerpc port

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Message-ID: <20120823152331.GF3570@redhat.com>
Date:	Thu, 23 Aug 2012 17:23:31 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Alex Shi <alex.shi@...el.com>,
	Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Don Morris <don.morris@...com>,
	Tony Breeds <tbreeds@....ibm.com>,
	Kumar Gala <galak@...nel.crashing.org>
Subject: Re: [PATCH 33/36] autonuma: powerpc port

Hi Benjamin,

On Thu, Aug 23, 2012 at 03:11:00PM +1000, Benjamin Herrenschmidt wrote:
> Basically PROT_NONE turns into _PAGE_PRESENT without _PAGE_USER for us.

Maybe the simplest is to implement pte_numa as !_PAGE_USER too. No
need to clear the _PAGE_PRESENT bit and to alter pte_present() if
clearing _PAGE_USER already achieves it.

It should be trivial to add the vma parameter to pte_numa(pte, vma) so
you can implement pte_numa by checking the vma->vm_page_prot in the
inline pte_numa function, to be able to tell if it's a real prot none
(in which case pte_numa return false) or if it's the NUMA hinting page
fault. In the latter case pte_numa will return true.

> However, the embedded ppc situation is more interesting... and it looks
> like it is indeed broken, meaning that a user can coerce the kernel into
> accessing PROT_NONE on its behalf with copy_from_user & co (though read
> only really).
> 
> Looks like the SW TLB handlers used on embedded should also check
> whether the address is a user or kernel address, and enforce _PAGE_USER
> in the former case. They might have done in the past, it's possible that
> it's code we lost, but as it is, it's broken.
> 
> The case of HW loaded TLB embedded will need a different definition of
> PAGE_NONE as well I suspect. Kumar, can you have a look ?

Even if we can't track copy-user accesses with the NUMA
hinting page faults, AUTONUMA should still work fairly well. The
flakey PROTNONE on embedded, is more a problem in itself than it would
be for pte_numa on embedded.

OTOH AutoNUMA working on embedded isn't important so it may be just
better to disable it until !_PAGE_USER is reliable.

> I wasn't especially thinking of ppc32... there's also hash64-4k or
> embedded 64... Also pgtable.h is common, so all those added uses of
> _PAGE_NUMA_PTE to static inline functions are going to break the build
> unless _PAGE_NUMA_PTE is #defined to 0 when not used (we do that for a
> bunch of bits in pte-common.h already).

It'd be actually worse if it would build ;). But I guess using
_PAGE_USER to implement pte_numa will solve the problem for 4k page
size too.

We can discuss this during kernel summit ;).

Thanks a lot!
Andrea
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