[PATCH v4 6/8] mm: make clear_huge_page cache clear only around the fault address

["Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

Clearing a 2MB huge page will typically blow away several levels
of CPU caches. To avoid this only cache clear the 4K area
around the fault address and use a cache avoiding clears
for the rest of the 2MB area.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/memory.c |   37 +++++++++++++++++++++++++++++--------
 1 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index dfc179b..625ca33 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3969,18 +3969,32 @@ EXPORT_SYMBOL(might_fault);
 #endif
 
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
+
+#ifndef ARCH_HAS_USER_NOCACHE
+#define ARCH_HAS_USER_NOCACHE 0
+#endif
+
+#if ARCH_HAS_USER_NOCACHE == 0
+#define clear_user_highpage_nocache clear_user_highpage
+#endif
+
 static void clear_gigantic_page(struct page *page,
-				unsigned long addr,
-				unsigned int pages_per_huge_page)
+		unsigned long haddr, unsigned long fault_address,
+		unsigned int pages_per_huge_page)
 {
 	int i;
 	struct page *p = page;
+	unsigned long vaddr;
+	int target = (fault_address - haddr) >> PAGE_SHIFT;
 
 	might_sleep();
-	for (i = 0; i < pages_per_huge_page;
-	     i++, p = mem_map_next(p, page, i)) {
+	for (i = 0, vaddr = haddr; i < pages_per_huge_page;
+			i++, p = mem_map_next(p, page, i), vaddr += PAGE_SIZE) {
 		cond_resched();
-		clear_user_highpage(p, addr + i * PAGE_SIZE);
+		if (!ARCH_HAS_USER_NOCACHE  || i == target)
+			clear_user_highpage(p, vaddr);
+		else
+			clear_user_highpage_nocache(p, vaddr);
 	}
 }
 void clear_huge_page(struct page *page,
@@ -3988,16 +4002,23 @@ void clear_huge_page(struct page *page,
 		     unsigned int pages_per_huge_page)
 {
 	int i;
+	unsigned long vaddr;
+	int target = (fault_address - haddr) >> PAGE_SHIFT;
 
 	if (unlikely(pages_per_huge_page > MAX_ORDER_NR_PAGES)) {
-		clear_gigantic_page(page, haddr, pages_per_huge_page);
+		clear_gigantic_page(page, haddr, fault_address,
+				pages_per_huge_page);
 		return;
 	}
 
 	might_sleep();
-	for (i = 0; i < pages_per_huge_page; i++) {
+	for (i = 0, vaddr = haddr; i < pages_per_huge_page;
+			i++, page++, vaddr += PAGE_SIZE) {
 		cond_resched();
-		clear_user_highpage(page + i, haddr + i * PAGE_SIZE);
+		if (!ARCH_HAS_USER_NOCACHE || i == target)
+			clear_user_highpage(page, vaddr);
+		else
+			clear_user_highpage_nocache(page, vaddr);
 	}
 }
 
-- 
1.7.7.6