mm: introduce page_shift()

Replace PAGE_SHIFT + compound_order(page) with the new page_shift()
function.  Minor improvements in readability.

[akpm@linux-foundation.org: fix build in tce_page_is_contained()]
  Link: http://lkml.kernel.org/r/201907241853.yNQTrJWd%25lkp@intel.com
Link: http://lkml.kernel.org/r/20190721104612.19120-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/powerpc/mm/book3s64/iommu_api.c

@@ -129,11 +129,8 @@ static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
 		 * Allow to use larger than 64k IOMMU pages. Only do that
 		 * if we are backed by hugetlb.
 		 */
-		if ((mem->pageshift > PAGE_SHIFT) && PageHuge(page)) {
-			struct page *head = compound_head(page);
-
-			pageshift = compound_order(head) + PAGE_SHIFT;
-		}
+		if ((mem->pageshift > PAGE_SHIFT) && PageHuge(page))
+			pageshift = page_shift(compound_head(page));
 		mem->pageshift = min(mem->pageshift, pageshift);
 		/*
 		 * We don't need struct page reference any more, switch

drivers/vfio/vfio_iommu_spapr_tce.c

@@ -176,21 +176,21 @@ static long tce_iommu_register_pages(struct tce_container *container,
 }
 
 static bool tce_page_is_contained(struct mm_struct *mm, unsigned long hpa,
-		unsigned int page_shift)
+		unsigned int it_page_shift)
 {
 	struct page *page;
 	unsigned long size = 0;
 
-	if (mm_iommu_is_devmem(mm, hpa, page_shift, &size))
-		return size == (1UL << page_shift);
+	if (mm_iommu_is_devmem(mm, hpa, it_page_shift, &size))
+		return size == (1UL << it_page_shift);
 
 	page = pfn_to_page(hpa >> PAGE_SHIFT);
 	/*
 	 * Check that the TCE table granularity is not bigger than the size of
 	 * a page we just found. Otherwise the hardware can get access to
 	 * a bigger memory chunk that it should.
 	 */
-	return (PAGE_SHIFT + compound_order(compound_head(page))) >= page_shift;
+	return page_shift(compound_head(page)) >= it_page_shift;
 }
 
 static inline bool tce_groups_attached(struct tce_container *container)

include/linux/mm.h

@@ -811,6 +811,12 @@ static inline unsigned long page_size(struct page *page)
 	return PAGE_SIZE << compound_order(page);
 }
 
+/* Returns the number of bits needed for the number of bytes in a page */
+static inline unsigned int page_shift(struct page *page)
+{
+	return PAGE_SHIFT + compound_order(page);
+}
+
 void free_compound_page(struct page *page);
 
 #ifdef CONFIG_MMU