[PATCH v10 3/8] mm: introduce clear_pages() and clear_user_pages()

[Ankur Arora <ankur.a.arora@oracle.com>]

Introduce clear_pages(), to be overridden by architectures that
support more efficient clearing of consecutive pages.

Also introduce clear_user_pages(), however, we will not expect this
function to be overridden anytime soon. 

As we do for clear_user_page(). define clear_user_pages() only if the
architecture does not define clear_user_highpage().

That is because if the architecture does define clear_user_highpage(),
then it likely needs some flushing magic when clearing user pages or
highpages. This means we can get away without defining clear_user_pages(),
since, much like its single page sibling, its only potential user is the
generic clear_user_highpages() which should instead be using
clear_user_highpage().

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---

Note:
  - reorganize based on Christophe Leroy's suggestion.
  - Dropped David's ack.

 include/linux/highmem.h | 33 +++++++++++++++++++++++++++++++++
 include/linux/mm.h      | 20 ++++++++++++++++++++
 2 files changed, 53 insertions(+)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 9187bfaa709d..92aa1053c9c1 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -217,6 +217,39 @@ static inline void clear_user_page(void *addr, unsigned long vaddr, struct page
 }
 #endif
 
+/**
+ * clear_user_pages() - clear a page range to be mapped to user space
+ * @addr: start address
+ * @vaddr: start address of the user mapping
+ * @page: start page
+ * @npages: number of pages
+ *
+ * Assumes that the region (@addr, +@npages) has been validated
+ * already so this does no exception handling.
+ *
+ * If the architecture provides a clear_user_page(), use that;
+ * otherwise, we can safely use clear_pages().
+ */
+static inline void clear_user_pages(void *addr, unsigned long vaddr,
+		struct page *page, unsigned int npages)
+{
+
+#ifdef clear_user_page
+	do {
+		clear_user_page(addr, vaddr, page);
+		addr += PAGE_SIZE;
+		vaddr += PAGE_SIZE;
+		page++;
+	} while (--npages);
+#else
+	/*
+	 * Prefer clear_pages() to allow for architectural optimizations
+	 * when operating on contiguous page ranges.
+	 */
+	clear_pages(addr, npages);
+#endif
+}
+
 /**
  * clear_user_highpage() - clear a page to be mapped to user space
  * @page: start page
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 15076261d0c2..12106ebf1a50 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4194,6 +4194,26 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
 				unsigned int order) {}
 #endif	/* CONFIG_DEBUG_PAGEALLOC */
 
+#ifndef clear_pages
+/**
+ * clear_pages() - clear a page range for kernel-internal use.
+ * @addr: start address
+ * @npages: number of pages
+ *
+ * Use clear_user_pages() instead when clearing a page range to be
+ * mapped to user space.
+ *
+ * Does absolutely no exception handling.
+ */
+static inline void clear_pages(void *addr, unsigned int npages)
+{
+	do {
+		clear_page(addr);
+		addr += PAGE_SIZE;
+	} while (--npages);
+}
+#endif
+
 #ifdef __HAVE_ARCH_GATE_AREA
 extern struct vm_area_struct *get_gate_vma(struct mm_struct *mm);
 extern int in_gate_area_no_mm(unsigned long addr);
-- 
2.31.1