[RFC PATCH 1/2] thp: Introduce arch_(un)block_thp_split

[Steve Capper]

In kernel/futex.c an assumption is made in the THP tail pinning code
that disabling the irqs is sufficient to block the THP splitting code
from completing.

Architectures such as ARM do hardware TLB broadcasts so disabling the
irqs will not prevent a THP split from completing.

This patch introduces: arch_block_thp_split and arch_unblock_thp_split,
and provides stock implementations that observe the original irq enable
and disable behaviour.

One can then define these macros themselves to implement architecture
specific behaviour.

Signed-off-by: Steve Capper <steve.capper at linaro.org>
---
 include/linux/huge_mm.h | 16 ++++++++++++++++
 kernel/futex.c          |  6 +++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 3935428..35fb742 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -177,6 +177,14 @@ static inline struct page *compound_trans_head(struct page *page)
 extern int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				unsigned long addr, pmd_t pmd, pmd_t *pmdp);
 
+#ifndef arch_block_thp_split
+#define arch_block_thp_split(mm)	local_irq_disable()
+#endif
+
+#ifndef arch_unblock_thp_split
+#define arch_unblock_thp_split(mm)	local_irq_enable()
+#endif
+
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
 #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
 #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
@@ -227,6 +235,14 @@ static inline int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_str
 	return 0;
 }
 
+#ifndef arch_block_thp_split
+#define arch_block_thp_split(mm)	do { } while (0)
+#endif
+
+#ifndef arch_unblock_thp_split
+#define arch_unblock_thp_split(mm)	do { } while (0)
+#endif
+
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 #endif /* _LINUX_HUGE_MM_H */
diff --git a/kernel/futex.c b/kernel/futex.c
index c3a1a55..e016ff2 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -287,7 +287,7 @@ again:
 	if (unlikely(PageTail(page))) {
 		put_page(page);
 		/* serialize against __split_huge_page_splitting() */
-		local_irq_disable();
+		arch_block_thp_split(mm);
 		if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) {
 			page_head = compound_head(page);
 			/*
@@ -304,9 +304,9 @@ again:
 				get_page(page_head);
 				put_page(page);
 			}
-			local_irq_enable();
+			arch_unblock_thp_split(mm);
 		} else {
-			local_irq_enable();
+			arch_unblock_thp_split(mm);
 			goto again;
 		}
 	}
-- 
1.8.1.4