[PATCHv6 01/22] mm: implement zero_huge_user_segment and friends

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Let's add helpers to clear huge page segment(s). They provide the same
functionallity as zero_user_segment and zero_user, but for huge pages.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 include/linux/mm.h | 18 ++++++++++++++++++
 mm/memory.c        | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8b6e55ee88..a7b7e62930 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1809,9 +1809,27 @@ extern void dump_page(struct page *page);
 extern void clear_huge_page(struct page *page,
 			    unsigned long addr,
 			    unsigned int pages_per_huge_page);
+extern void zero_huge_user_segment(struct page *page,
+		unsigned start, unsigned end);
+static inline void zero_huge_user(struct page *page,
+		unsigned start, unsigned len)
+{
+	zero_huge_user_segment(page, start, start + len);
+}
 extern void copy_user_huge_page(struct page *dst, struct page *src,
 				unsigned long addr, struct vm_area_struct *vma,
 				unsigned int pages_per_huge_page);
+#else
+static inline void zero_huge_user_segment(struct page *page,
+		unsigned start, unsigned end)
+{
+	BUILD_BUG();
+}
+static inline void zero_huge_user(struct page *page,
+		unsigned start, unsigned len)
+{
+	BUILD_BUG();
+}
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
 
 #ifdef CONFIG_DEBUG_PAGEALLOC
diff --git a/mm/memory.c b/mm/memory.c
index ca00039471..e5f74cd634 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4291,6 +4291,42 @@ void clear_huge_page(struct page *page,
 	}
 }
 
+void zero_huge_user_segment(struct page *page, unsigned start, unsigned end)
+{
+	int i;
+	unsigned start_idx, end_idx;
+	unsigned start_off, end_off;
+
+	BUG_ON(end < start);
+
+	might_sleep();
+
+	if (start == end)
+		return;
+
+	start_idx = start >> PAGE_SHIFT;
+	start_off = start & ~PAGE_MASK;
+	end_idx = (end - 1) >> PAGE_SHIFT;
+	end_off = ((end - 1) & ~PAGE_MASK) + 1;
+
+	/*
+	 * if start and end are on the same small page we can call
+	 * zero_user_segment() once and save one kmap_atomic().
+	 */
+	if (start_idx == end_idx)
+		return zero_user_segment(page + start_idx, start_off, end_off);
+
+	/* zero the first (possibly partial) page */
+	zero_user_segment(page + start_idx, start_off, PAGE_SIZE);
+	for (i = start_idx + 1; i < end_idx; i++) {
+		cond_resched();
+		clear_highpage(page + i);
+		flush_dcache_page(page + i);
+	}
+	/* zero the last (possibly partial) page */
+	zero_user_segment(page + end_idx, 0, end_off);
+}
+
 static void copy_user_gigantic_page(struct page *dst, struct page *src,
 				    unsigned long addr,
 				    struct vm_area_struct *vma,
-- 
1.8.4.rc3