Linux 6.12: PG_error flag was removed

torvalds/linux@09022bc196d2 removes the flag, and the corresponding
SetPageError() and ClearPageError() macros, with no replacement offered.

Going back through the upstream history, use of this flag has been
gradually removed over the last year as part of the long tail of
converting everything to folios. Interesting tidbit comments from
torvalds/linux@29e9412b250e and torvalds/linux@420e05d0de18 suggest that
this flag has not been used meaningfully since page writeback failures
started being recorded in errseq_t instead (the whole "fsyncgate" thing,
~2017, around torvalds/linux@8ed1e46aaf1b).

Given that, it's possible that since perhaps Linux 4.13 we haven't been
getting anything by setting the flag. I don't know if that's true and/or
if there's something we should be doing instead, but my gut feel is that
its probably fine we only use the page cache as a proxy to allow mmap()
to work, rather than backing IO with it.

As such, I'm expecting that removing this will do no harm, but I'm
leaving it in for older kernels to maintain status quo, and if there is
an overall better way, that is left for a future change.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#16582

config/kernel-mm-page-flags.m4

@@ -0,0 +1,24 @@
+AC_DEFUN([ZFS_AC_KERNEL_SRC_MM_PAGE_FLAG_ERROR], [
+	ZFS_LINUX_TEST_SRC([mm_page_flag_error], [
+		#include <linux/page-flags.h>
+
+		static enum pageflags
+		    test_flag __attribute__((unused)) = PG_error;
+	])
+])
+AC_DEFUN([ZFS_AC_KERNEL_MM_PAGE_FLAG_ERROR], [
+	AC_MSG_CHECKING([whether PG_error flag is available])
+	ZFS_LINUX_TEST_RESULT([mm_page_flag_error], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_MM_PAGE_FLAG_ERROR, 1, [PG_error flag is available])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_SRC_MM_PAGE_FLAGS], [
+	ZFS_AC_KERNEL_SRC_MM_PAGE_FLAG_ERROR
+])
+AC_DEFUN([ZFS_AC_KERNEL_MM_PAGE_FLAGS], [
+	ZFS_AC_KERNEL_MM_PAGE_FLAG_ERROR
+])

config/kernel.m4

@@ -170,6 +170,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_PROC_HANDLER_CTL_TABLE_CONST
 	ZFS_AC_KERNEL_SRC_COPY_SPLICE_READ
 	ZFS_AC_KERNEL_SRC_SYNC_BDEV
+	ZFS_AC_KERNEL_SRC_MM_PAGE_FLAGS
 	ZFS_AC_KERNEL_SRC_MM_PAGE_SIZE
 	ZFS_AC_KERNEL_SRC_MM_PAGE_MAPPING
 	ZFS_AC_KERNEL_SRC_FILE
@@ -325,6 +326,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_PROC_HANDLER_CTL_TABLE_CONST
 	ZFS_AC_KERNEL_COPY_SPLICE_READ
 	ZFS_AC_KERNEL_SYNC_BDEV
+	ZFS_AC_KERNEL_MM_PAGE_FLAGS
 	ZFS_AC_KERNEL_MM_PAGE_SIZE
 	ZFS_AC_KERNEL_MM_PAGE_MAPPING
 	ZFS_AC_KERNEL_1ARG_ASSIGN_STR

include/os/linux/kernel/linux/mm_compat.h

@@ -40,4 +40,18 @@
 #define	page_mapping(p) folio_mapping(page_folio(p))
 #endif
 
+/*
+ * 6.12 removed PG_error, SetPageError and ClearPageError, with no direct
+ * replacement, because page writeback errors are recorded elsewhere. Since we
+ * only use the page cache to assist with mmap(), never directly backing it
+ * with IO, it shouldn't be possible for this condition to occur on our pages
+ * anyway, even if this is the right way to report it. So it should be safe
+ * to remove, but for avoidance of doubt, we make it a no-op on 6.12 and leave
+ * it for everything else.
+ */
+#ifndef HAVE_MM_PAGE_FLAG_ERROR
+#define	SetPageError(p)		do {} while (0)
+#define	ClearPageError(p)	do {} while (0)
+#endif
+
 #endif /* _ZFS_MM_COMPAT_H */

module/os/linux/zfs/zfs_znode.c

@@ -61,6 +61,7 @@
 #include <sys/sa.h>
 #include <sys/zfs_sa.h>
 #include <sys/zfs_stat.h>
+#include <linux/mm_compat.h>
 
 #include "zfs_prop.h"
 #include "zfs_comutil.h"