[RFC PATCH v2 2/3] xpfo: Only put previous userspace pages into the hot cache

[Juerg Haefliger]

Allocating a page to userspace that was previously allocated to the
kernel requires an expensive TLB shootdown. To minimize this, we only
put non-kernel pages into the hot cache to favor their allocation.

Signed-off-by: Juerg Haefliger <juerg.haefliger@xxxxxxx>
---
 include/linux/xpfo.h | 2 ++
 mm/page_alloc.c      | 8 +++++++-
 mm/xpfo.c            | 8 ++++++++
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/linux/xpfo.h b/include/linux/xpfo.h
index 77187578ca33..077d1cfadfa2 100644
--- a/include/linux/xpfo.h
+++ b/include/linux/xpfo.h
@@ -24,6 +24,7 @@ extern void xpfo_alloc_page(struct page *page, int order, gfp_t gfp);
 extern void xpfo_free_page(struct page *page, int order);
 
 extern bool xpfo_page_is_unmapped(struct page *page);
+extern bool xpfo_page_is_kernel(struct page *page);
 
 #else /* !CONFIG_XPFO */
 
@@ -33,6 +34,7 @@ static inline void xpfo_alloc_page(struct page *page, int order, gfp_t gfp) { }
 static inline void xpfo_free_page(struct page *page, int order) { }
 
 static inline bool xpfo_page_is_unmapped(struct page *page) { return false; }
+static inline bool xpfo_page_is_kernel(struct page *page) { return false; }
 
 #endif /* CONFIG_XPFO */
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0241c8a7e72a..83404b41e52d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2421,7 +2421,13 @@ void free_hot_cold_page(struct page *page, bool cold)
 	}
 
 	pcp = &this_cpu_ptr(zone->pageset)->pcp;
-	if (!cold)
+	/*
+	 * XPFO: Allocating a page to userspace that was previously allocated
+	 * to the kernel requires an expensive TLB shootdown. To minimize this,
+	 * we only put non-kernel pages into the hot cache to favor their
+	 * allocation.
+	 */
+	if (!cold && !xpfo_page_is_kernel(page))
 		list_add(&page->lru, &pcp->lists[migratetype]);
 	else
 		list_add_tail(&page->lru, &pcp->lists[migratetype]);
diff --git a/mm/xpfo.c b/mm/xpfo.c
index ddb1be05485d..f8dffda0c961 100644
--- a/mm/xpfo.c
+++ b/mm/xpfo.c
@@ -203,3 +203,11 @@ inline bool xpfo_page_is_unmapped(struct page *page)
 
 	return test_bit(PAGE_EXT_XPFO_UNMAPPED, &lookup_page_ext(page)->flags);
 }
+
+inline bool xpfo_page_is_kernel(struct page *page)
+{
+	if (!static_branch_unlikely(&xpfo_inited))
+		return false;
+
+	return test_bit(PAGE_EXT_XPFO_KERNEL, &lookup_page_ext(page)->flags);
+}
-- 
2.9.3