[PATCH v11 3/8] highmem: introduce clear_user_highpages()

[Ankur Arora]

Define clear_user_highpages() which uses the range clearing primitive,
clear_user_pages(). We can safely use this when CONFIG_HIGHMEM is
disabled and if the architecture does not have clear_user_highpage.

The first is needed to ensure that contiguous page ranges stay
contiguous which precludes intermediate maps via HIGMEM.
The second, because if the architecture has clear_user_highpage(),
it likely needs flushing magic when clearing the page, magic that
we aren't privy to.

For both of those cases, just fallback to a loop around
clear_user_highpage().

Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 include/linux/highmem.h | 45 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 019ab7d8c841..af03db851a1d 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -251,7 +251,14 @@ static inline void clear_user_pages(void *addr, unsigned long vaddr,
 #endif
 }
 
-/* when CONFIG_HIGHMEM is not set these will be plain clear/copy_page */
+/**
+ * clear_user_highpage() - clear a page to be mapped to user space
+ * @page: start page
+ * @vaddr: start address of the user mapping
+ *
+ * With !CONFIG_HIGHMEM this (and the copy_user_highpage() below) will
+ * be plain clear_user_page() (and copy_user_page()).
+ */
 static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
 {
 	void *addr = kmap_local_page(page);
@@ -260,6 +267,42 @@ static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
 }
 #endif /* clear_user_highpage */
 
+/**
+ * clear_user_highpages() - clear a page range to be mapped to user space
+ * @page: start page
+ * @vaddr: start address of the user mapping
+ * @npages: number of pages
+ *
+ * Assumes that all the pages in the region (@page, +@npages) are valid
+ * so this does no exception handling.
+ */
+static inline void clear_user_highpages(struct page *page, unsigned long vaddr,
+					unsigned int npages)
+{
+
+#if defined(clear_user_highpage) || defined(CONFIG_HIGHMEM)
+	/*
+	 * An architecture defined clear_user_highpage() implies special
+	 * handling is needed.
+	 *
+	 * So we use that or, the generic variant if CONFIG_HIGHMEM is
+	 * enabled.
+	 */
+	do {
+		clear_user_highpage(page, vaddr);
+		vaddr += PAGE_SIZE;
+		page++;
+	} while (--npages);
+#else
+
+	/*
+	 * Prefer clear_user_pages() to allow for architectural optimizations
+	 * when operating on contiguous page ranges.
+	 */
+	clear_user_pages(page_address(page), vaddr, page, npages);
+#endif
+}
+
 #ifndef vma_alloc_zeroed_movable_folio
 /**
  * vma_alloc_zeroed_movable_folio - Allocate a zeroed page for a VMA.
-- 
2.31.1