linux-kernel - [PATCH 05/36] autonuma: teach gup_fast about pmd

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Date:	Wed, 22 Aug 2012 16:58:49 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Alex Shi <alex.shi@...el.com>,
	Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Don Morris <don.morris@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: [PATCH 05/36] autonuma: teach gup_fast about pmd_numa

In the special "pmd" mode of knuma_scand
(/sys/kernel/mm/autonuma/knuma_scand/pmd == 1), 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@...hat.com>
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
 arch/x86/mm/gup.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)

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))
--
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