highmem: add folio_test_partial_kmap()

In commit c749d9b7ebbc ("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: 00cdf76012ab ("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>

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 5c6bea8..c698f84 100644 (file)
--- a/include/linux/highmem.h
+++ b/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

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index e6a21b6..3b814ce 100644 (file)
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -615,6 +615,13 @@ 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)
 {