[PATCH 2/2] highmem: Add memcpy_from_folio()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

This is the folio equivalent of memcpy_from_page(), but it handles large
highmem folios.  It may be a little too big to inline on systems that
have CONFIG_HIGHMEM enabled but on systems we actually care about almost
all the code will be eliminated.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/highmem.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index ec39f544113d..d47f4a09f2fa 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -536,6 +536,34 @@ static inline void memcpy_to_folio(struct folio *folio, size_t offset,
 	flush_dcache_folio(folio);
 }
 
+/**
+ * memcpy_from_folio - Copy a range of bytes from a folio
+ * @to: The memory to copy to.
+ * @folio: The folio to read from.
+ * @offset: The first byte in the folio to read.
+ * @len: The number of bytes to copy.
+ */
+static inline void memcpy_from_folio(char *to, struct folio *folio,
+		size_t offset, size_t len)
+{
+	size_t n = len;
+
+	VM_BUG_ON(offset + len > folio_size(folio));
+
+	if (folio_test_highmem(folio))
+		n = min(len, PAGE_SIZE - offset_in_page(offset));
+	for (;;) {
+		char *from = kmap_local_folio(folio, offset);
+		memcpy(to, from, n);
+		kunmap_local(from);
+		if (!folio_test_highmem(folio) || n == len)
+			break;
+		offset += n;
+		len -= n;
+		n = min(len, PAGE_SIZE);
+	}
+}
+
 static inline void put_and_unmap_page(struct page *page, void *addr)
 {
 	kunmap_local(addr);
-- 
2.39.2