Subject: Re: [PATCH -V2 3/5] mm: Move change_prot_numa outside CONFIG_ARCH_USES_NUMA_PROT_NONE

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 05 Dec 2013 10:48:13 +0530

Adding Mel and Rik to cc:

Benjamin Herrenschmidt <[email-masked]> writes:

> On Mon, 2013-11-18 at 14:58 +0530, Aneesh Kumar K.V wrote:
>> From: "Aneesh Kumar K.V" <[email-masked]>
>> 
>> change_prot_numa should work even if _PAGE_NUMA != _PAGE_PROTNONE.
>> On archs like ppc64 that don't use _PAGE_PROTNONE and also have
>> a separate page table outside linux pagetable, we just need to
>> make sure that when calling change_prot_numa we flush the
>> hardware page table entry so that next page access  result in a numa
>> fault.
>
> That patch doesn't look right...
>
> You are essentially making change_prot_numa() do whatever it does (which
> I don't completely understand) *for all architectures* now, whether they
> have CONFIG_ARCH_USES_NUMA_PROT_NONE or not ... So because you want that
> behaviour on powerpc book3s64, you change everybody.
>
> Is that correct ?

Yes. 

>
> Also what exactly is that doing, can you explain ? From what I can see,
> it calls back into the core of mprotect to change the protection to
> vma->vm_page_prot, which I would have expected is already the protection
> there, with the added "prot_numa" flag passed down.

it set the _PAGE_NUMA bit. Now we also want to make sure that when
we set _PAGE_NUMA, we would get a pagefault on that so that we can track
that fault as a numa fault. To ensure that, we had the below BUILD_BUG

	BUILD_BUG_ON(_PAGE_NUMA != _PAGE_PROTNONE);

But other than that the function doesn't really have any dependency on
_PAGE_PROTNONE. The only requirement is when we set _PAGE_NUMA, the
architecture should do enough to ensure that we get a page fault. Now on
ppc64 we does that by clearlying hpte entry and also clearing
_PAGE_PRESENT. Since we have _PAGE_PRESENT cleared hash_page will return
1 and we get to page fault handler.

>
> Your changeset comment says "On archs like ppc64 [...] we just need to
> make sure that when calling change_prot_numa we flush the
> hardware page table entry so that next page access  result in a numa
> fault."
>
> But change_prot_numa() does a lot more than that ... it does
> pte_mknuma(), do we need it ? I assume we do or we wouldn't have added
> that PTE bit to begin with...
>
> Now it *might* be allright and it might be that no other architecture
> cares anyway etc... but I need at least some mm folks to ack on that
> patch before I can take it because it *will* change behaviour of other
> architectures.
>

Ok, I can move the changes below #ifdef CONFIG_NUMA_BALANCING ? We call
change_prot_numa from task_numa_work and queue_pages_range(). The later
may be an issue. So doing the below will help ?

-#ifdef CONFIG_ARCH_USES_NUMA_PROT_NONE
+#ifdef CONFIG_NUMA_BALANCING

-aneesh