On Mon, Mar 31, 2014 at 5:26 AM, Mel Gorman <mgorman@xxxxxxx> wrote:
>
> Ok, so how do you suggest that _PAGE_NUMA could have been implemented
> that did *not* use _PAGE_PROTNONE on x86, trapped a fault and was not
> expensive as hell to handle?
So on x86, the obvious model is to use another bit. We've got several.
The _PAGE_NUMA case only matters for when _PAGE_PRESENT is clear, and
when that bit is clear the hardware doesn't care about any of the
other bits. Currently we use:
#define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL
#define _PAGE_BIT_FILE _PAGE_BIT_DIRTY
which are bits 8 and 6 respectively, afaik.
and the only rule is that (a) we should *not* use a bit we already use
when the page is not present (since that is ambiguous!) and (b) we
should *not* use a bit that is used by the swap index cases. I think
bit 7 should work, but maybe I missed something.
Can somebody tell me why _PAGE_NUMA is *not* that bit seven? Make
"pte_present()" on x86 just check all of the present/numa/protnone
bits, and if any of them is set, it's a "present" page.
Now, unlike x86, some other architectures do *not* have free bits, so
there may be problems elsewhere.
Linus
--
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>