Fix build for Linux 6.18 with PowerPC/RISC-V kernels. (#18145)

The macro 'flush_dcache_page(...)' modifies the page flags, but in Linux
6.18 the type of the page flags changed from 'unsigned long' to the
struct type 'memdesc_flags_t' with a single member 'f' which is the page
flags field.

Signed-off-by: Erik Larsson <catacombae@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

config/kernel-mm-page-flags.m4

@@ -17,9 +17,36 @@ AC_DEFUN([ZFS_AC_KERNEL_MM_PAGE_FLAG_ERROR], [
 	])
 ])
 
+dnl #
+dnl # Linux 6.18+ uses a struct typedef (memdesc_flags_t) instead of an
+dnl # 'unsigned long' for the 'flags' field in 'struct page'.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_MM_PAGE_FLAGS_STRUCT], [
+	ZFS_LINUX_TEST_SRC([mm_page_flags_struct], [
+		#include <linux/mm.h>
+
+		static const struct page p __attribute__ ((unused)) = {
+			.flags = { .f = 0 }
+		};
+	],[])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_MM_PAGE_FLAGS_STRUCT], [
+	AC_MSG_CHECKING([whether 'flags' in 'struct page' is a struct])
+	ZFS_LINUX_TEST_RESULT([mm_page_flags_struct], [
+		AC_MSG_RESULT([yes])
+		AC_DEFINE(HAVE_MM_PAGE_FLAGS_STRUCT, 1,
+			['flags' in 'struct page' is a struct])
+	],[
+		AC_MSG_RESULT([no])
+	])
+])
+
 AC_DEFUN([ZFS_AC_KERNEL_SRC_MM_PAGE_FLAGS], [
 	ZFS_AC_KERNEL_SRC_MM_PAGE_FLAG_ERROR
+	ZFS_AC_KERNEL_SRC_MM_PAGE_FLAGS_STRUCT
 ])
 AC_DEFUN([ZFS_AC_KERNEL_MM_PAGE_FLAGS], [
 	ZFS_AC_KERNEL_MM_PAGE_FLAG_ERROR
+	ZFS_AC_KERNEL_MM_PAGE_FLAGS_STRUCT
 ])

include/os/linux/kernel/linux/dcache_compat.h

@@ -34,6 +34,17 @@
 
 #define	d_alias			d_u.d_alias
 
+#ifdef HAVE_MM_PAGE_FLAGS_STRUCT
+/*
+ * Starting from Linux 6.18, the 'flags' field in 'struct page' is defined
+ * to a struct ('memdesc_flags_t' typedef) instead of an unsigned long for
+ * improved typesafety.
+ */
+#define	page_flags flags.f
+#else
+#define	page_flags flags
+#endif
+
 /*
  * Starting from Linux 5.13, flush_dcache_page() becomes an inline function
  * and under some configurations, may indirectly referencing GPL-only
@@ -44,8 +55,8 @@
 #include <linux/simd_powerpc.h>
 #define	flush_dcache_page(page)	do {					\
 		if (!cpu_has_feature(CPU_FTR_COHERENT_ICACHE) &&	\
-		    test_bit(PG_dcache_clean, &(page)->flags))		\
-			clear_bit(PG_dcache_clean, &(page)->flags);	\
+		    test_bit(PG_dcache_clean, &(page)->page_flags))	\
+			clear_bit(PG_dcache_clean, &(page)->page_flags);\
 	} while (0)
 #endif
 /*
@@ -55,8 +66,8 @@
  */
 #if defined __riscv && defined HAVE_FLUSH_DCACHE_PAGE_GPL_ONLY
 #define	flush_dcache_page(page)	do {					\
-		if (test_bit(PG_dcache_clean, &(page)->flags))		\
-			clear_bit(PG_dcache_clean, &(page)->flags);	\
+		if (test_bit(PG_dcache_clean, &(page)->page_flags))	\
+			clear_bit(PG_dcache_clean, &(page)->page_flags);\
 	} while (0)
 #endif