On Mon, Apr 07, 2014 at 08:19:10PM +0400, Cyrill Gorcunov wrote: > On Mon, Apr 07, 2014 at 04:49:35PM +0100, Mel Gorman wrote: > > On Mon, Apr 07, 2014 at 04:32:39PM +0100, David Vrabel wrote: > > > On 07/04/14 16:10, Mel Gorman wrote: > > > > _PAGE_NUMA is currently an alias of _PROT_PROTNONE to trap NUMA hinting > > > > faults. As the bit is shared care is taken that _PAGE_NUMA is only used in > > > > places where _PAGE_PROTNONE could not reach but this still causes problems > > > > on Xen and conceptually difficult. > > > > > > The problem with Xen guests occurred because mprotect() /was/ confusing > > > PROTNONE mappings with _PAGE_NUMA and clearing the non-existant NUMA hints. > > > > I didn't bother spelling it out in case I gave the impression that I was > > blaming Xen for the problem. As the bit is now changes, does it help > > the Xen problem or cause another collision of some sort? There is no > > guarantee _PAGE_NUMA will remain as bit 62 but at worst it'll use bit 11 > > and NUMA_BALANCING will depend in !KMEMCHECK. > > Fwiw, we're using bit 11 for soft-dirty tracking, so i really hope worst case > never happen. (At the moment I'm trying to figure out if with this set > it would be possible to clean up ugly macros in pgoff_to_pte for 2 level pages). I had considered the soft-dirty tracking usage of the same bit. I thought I'd be able to swizzle around it or a further worst case of having soft-dirty and automatic NUMA balancing mutually exclusive. Unfortunately upon examination it's not obvious how to have both of them share a bit and I suspect any attempt to will break CRIU. In my current tree, NUMA_BALANCING cannot be set if MEM_SOFT_DIRTY which is not particularly satisfactory. Next on the list is examining if _PAGE_BIT_IOMAP can be used. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>