highmem: add folio_test_partial_kmap()

BugLink: https://bugs.launchpad.net/bugs/2115678

commit 97dfbbd135cb5e4426f37ca53a8fa87eaaa4e376 upstream.

In commit c749d9b ("iov_iter: fix copy_page_from_iter_atomic() if
KMAP_LOCAL_FORCE_MAP"), Hugh correctly noted that if KMAP_LOCAL_FORCE_MAP
is enabled, we must limit ourselves to PAGE_SIZE bytes per call to
kmap_local().  The same problem exists in memcpy_from_folio(),
memcpy_to_folio(), folio_zero_tail(), folio_fill_tail() and
memcpy_from_file_folio(), so add folio_test_partial_kmap() to do this more
succinctly.

Link: https://lkml.kernel.org/r/20250514170607.3000994-2-willy@infradead.org
Fixes: 00cdf76 ("mm: add memcpy_from_file_folio()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Mehmet Basaran <mehmet.basaran@canonical.com>

Author: Matthew Wilcox (Oracle)

include/linux/highmem.h

@@ -461,7 +461,7 @@ static inline void memcpy_from_folio(char *to, struct folio *folio,
 		const char *from = kmap_local_folio(folio, offset);
 		size_t chunk = len;
 
-		if (folio_test_highmem(folio) &&
+		if (folio_test_partial_kmap(folio) &&
 		    chunk > PAGE_SIZE - offset_in_page(offset))
 			chunk = PAGE_SIZE - offset_in_page(offset);
 		memcpy(to, from, chunk);
@@ -489,7 +489,7 @@ static inline void memcpy_to_folio(struct folio *folio, size_t offset,
 		char *to = kmap_local_folio(folio, offset);
 		size_t chunk = len;
 
-		if (folio_test_highmem(folio) &&
+		if (folio_test_partial_kmap(folio) &&
 		    chunk > PAGE_SIZE - offset_in_page(offset))
 			chunk = PAGE_SIZE - offset_in_page(offset);
 		memcpy(to, from, chunk);
@@ -522,7 +522,7 @@ static inline __must_check void *folio_zero_tail(struct folio *folio,
 {
 	size_t len = folio_size(folio) - offset;
 
-	if (folio_test_highmem(folio)) {
+	if (folio_test_partial_kmap(folio)) {
 		size_t max = PAGE_SIZE - offset_in_page(offset);
 
 		while (len > max) {
@@ -560,7 +560,7 @@ static inline void folio_fill_tail(struct folio *folio, size_t offset,
 
 	VM_BUG_ON(offset + len > folio_size(folio));
 
-	if (folio_test_highmem(folio)) {
+	if (folio_test_partial_kmap(folio)) {
 		size_t max = PAGE_SIZE - offset_in_page(offset);
 
 		while (len > max) {
@@ -597,7 +597,7 @@ static inline size_t memcpy_from_file_folio(char *to, struct folio *folio,
 	size_t offset = offset_in_folio(folio, pos);
 	char *from = kmap_local_folio(folio, offset);
 
-	if (folio_test_highmem(folio)) {
+	if (folio_test_partial_kmap(folio)) {
 		offset = offset_in_page(offset);
 		len = min_t(size_t, len, PAGE_SIZE - offset);
 	} else

include/linux/page-flags.h

@@ -578,6 +578,13 @@ FOLIO_FLAG(dropbehind, FOLIO_HEAD_PAGE)
 PAGEFLAG_FALSE(HighMem, highmem)
 #endif
 
+/* Does kmap_local_folio() only allow access to one page of the folio? */
+#ifdef CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP
+#define folio_test_partial_kmap(f)	true
+#else
+#define folio_test_partial_kmap(f)	folio_test_highmem(f)
+#endif
+
 #ifdef CONFIG_SWAP
 static __always_inline bool folio_test_swapcache(const struct folio *folio)
 {