linux-kernel - Re: [PATCH 03/10] mm: Convert p[te|md]_numa users to p[te|md]_protnone

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Date:	Tue, 02 Dec 2014 12:57:00 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Mel Gorman <mgorman@...e.de>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LinuxPPC-dev <linuxppc-dev@...ts.ozlabs.org>,
	Hugh Dickins <hughd@...gle.com>, Dave Jones <davej@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 03/10] mm: Convert p[te|md]_numa users to p[te|md]_protnone_numa

Benjamin Herrenschmidt <benh@...nel.crashing.org> writes:

> On Fri, 2014-11-21 at 13:57 +0000, Mel Gorman wrote:
>> Convert existing users of pte_numa and friends to the new helper. Note
>> that the kernel is broken after this patch is applied until the other
>> page table modifiers are also altered. This patch layout is to make
>> review easier.
>
> Aneesh, the removal of the DSISR_PROTFAULT checks, I wonder if we might
> break something here ... (I know, I asked for them to be removed :-)
>

That is the reason I converted that to a WARN_ON in later patch. 

> IE, we basically bounce all protection checks to the "normal" VMA
> protection checking, so far so good...
>
> But what about the subpage protection stuff ? Will that still work ?
>

I did look at that before. So if we had subpage access limitted, when we
take a fault for that subpage, we bail out early in hash_page_mm. (with
rc = 2). low_hash_fault handle that case directly. We will not end up
calling do_page_fault.

Now, hash_preload can possibly insert an hpte in hash page table even if
the access is not allowed by the pte permissions. But i guess even that
is ok. because we will fault again, end-up calling hash_page_mm where we
handle that part correctly.

-aneesh

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