On 11/06/2012 04:14 AM, Mel Gorman wrote:
From: Andrea Arcangeli <aarcange@xxxxxxxxxx> The objective of _PAGE_NUMA is to be able to trigger NUMA hinting page faults to identify the per NUMA node working set of the thread at runtime. Arming the NUMA hinting page fault mechanism works similarly to setting up a mprotect(PROT_NONE) virtual range: the present bit is cleared at the same time that _PAGE_NUMA is set, so when the fault triggers we can identify it as a NUMA hinting page fault. _PAGE_NUMA on x86 shares the same bit number of _PAGE_PROTNONE (but it could also use a different bitflag, it's up to the architecture to decide). It would be confusing to call the "NUMA hinting page faults" as "do_prot_none faults". They're different events and _PAGE_NUMA doesn't alter the semantics of mprotect(PROT_NONE) in any way. Sharing the same bitflag with _PAGE_PROTNONE in fact complicates things: it requires us to ensure the code paths executed by _PAGE_PROTNONE remains mutually exclusive to the code paths executed by _PAGE_NUMA at all times, to avoid _PAGE_NUMA and _PAGE_PROTNONE to step into each other toes. Because we want to be able to set this bitflag in any established pte or pmd (while clearing the present bit at the same time) without losing information, this bitflag must never be set when the pte and pmd are present, so the bitflag picked for _PAGE_NUMA usage, must not be used by the swap entry format. Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
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