highmem: introduce clear_user_highpages()

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().

Link: https://lkml.kernel.org/r/20260107072009.1615991-4-ankur.a.arora@oracle.com
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "Borislav Petkov (AMD)" <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Konrad Rzessutek Wilk <konrad.wilk@oracle.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Li Zhe <lizhe.67@bytedance.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Raghavendra K T <raghavendra.kt@amd.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 44 insertions, 1 deletions

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.