Messages in this thread | | | Date | Wed, 07 Nov 2012 05:48:30 -0500 | From | Rik van Riel <> | Subject | Re: [PATCH 08/19] mm: numa: Create basic numa page hinting infrastructure |
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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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