Re: [PATCH 08/19] mm: numa: Create basic numa page hinting infrastructure

[Rik van Riel]

On 11/07/2012 05:38 AM, Mel Gorman wrote:
> On Tue, Nov 06, 2012 at 01:58:26PM -0500, Rik van Riel wrote:
> > On 11/06/2012 04:14 AM, Mel Gorman wrote:
> > > Note: This patch started as "mm/mpol: Create special PROT_NONE
> > > 	infrastructure" and preserves the basic idea but steals *very*
> > > 	heavily from "autonuma: numa hinting page faults entry points" for
> > > 	the actual fault handlers without the migration parts.	The end
> > > 	result is barely recognisable as either patch so all Signed-off
> > > 	and Reviewed-bys are dropped. If Peter, Ingo and Andrea are ok with
> > > 	this version, I will re-add the signed-offs-by to reflect the history.
> > >
> > > In order to facilitate a lazy -- fault driven -- migration of pages, create
> > > a special transient PAGE_NUMA variant, we can then use the 'spurious'
> > > protection faults to drive our migrations from.
> > >
> > > Pages that already had an effective PROT_NONE mapping will not be detected
> >
> > The patch itself is good, but the changelog needs a little
> > fix. While you are defining _PAGE_NUMA to _PAGE_PROTNONE on
> > x86, this may be different on other architectures.
> >
> > Therefore, the changelog should refer to PAGE_NUMA, not
> > PROT_NONE.
>
> Fair point. I still want to record the point that PROT_NONE will not
> generate the faults though. How about this?
>
>      In order to facilitate a lazy -- fault driven -- migration of pages, create
>      a special transient PAGE_NUMA variant, we can then use the 'spurious'
>      protection faults to drive our migrations from.
>
>      The meaning of PAGE_NUMA depends on the architecture but on x86 it is
>      effectively PROT_NONE. In this case, PROT_NONE mappings will not be detected
>      to generate these 'spurious' faults for the simple reason that we cannot
>      distinguish them on their protection bits, see pte_numa(). This isn't
>      a problem since PROT_NONE (and possible PROT_WRITE with dirty tracking)
>      aren't used or are rare enough for us to not care about their placement.

Actual PROT_NONE mappings will not generate these NUMA faults
for the reason that the page fault code checks the permission
on the VMA (and will throw a segmentation fault on actual
PROT_NONE mappings), before it ever calls handle_mm_fault.

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