[PATCH 06/31] mm: numa: teach gup_fast about pmd_numa

[Mel Gorman]

From: Andrea Arcangeli <aarcange@xxxxxxxxxx>

When scanning pmds, the pmd may be of numa type (_PAGE_PRESENT not set),
however the pte might be present. Therefore, gup_pmd_range() must return
0 in this case to avoid losing a NUMA hinting page fault during gup_fast.

Note: gup_fast will skip over non present ptes (like numa types), so
no explicit check is needed for the pte_numa case. gup_fast will also
skip over THP when the trans huge pmd is non present. So, the pmd_numa
case will also be correctly skipped with no additional code changes
required.

Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 arch/x86/mm/gup.c |   13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index dd74e46..02c5ec5 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -163,8 +163,19 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
 		 * can't because it has irq disabled and
 		 * wait_split_huge_page() would never return as the
 		 * tlb flush IPI wouldn't run.
+		 *
+		 * The pmd_numa() check is needed because the code
+		 * doesn't check the _PAGE_PRESENT bit of the pmd if
+		 * the gup_pte_range() path is taken. NOTE: not all
+		 * gup_fast users will will access the page contents
+		 * using the CPU through the NUMA memory channels like
+		 * KVM does. So we're forced to trigger NUMA hinting
+		 * page faults unconditionally for all gup_fast users
+		 * even though NUMA hinting page faults aren't useful
+		 * to I/O drivers that will access the page with DMA
+		 * and not with the CPU.
 		 */
-		if (pmd_none(pmd) || pmd_trans_splitting(pmd))
+		if (pmd_none(pmd) || pmd_trans_splitting(pmd) || pmd_numa(pmd))
 			return 0;
 		if (unlikely(pmd_large(pmd))) {
 			if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
-- 
1.7.9.2

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