It seems to me that while (for example) on SPARC, it's not possible to
create a non-coherent mapping with mmap(), after we've done an mmap,
we can then use remap_file_pages() to create a mapping that no longer
aliases in the D-cache.
I have only compile-tested this patch. I don't have any SPARC hardware,
and my PA-RISC hardware hasn't been turned on in six years ... I noticed
this while wandering around looking at some other stuff.
diff --git a/mm/fremap.c b/mm/fremap.c
index 5bff081..01fc2e7 100644
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -19,6 +19,7 @@
#include <asm/mmu_context.h>
#include <asm/cacheflush.h>
+#include <asm/shmparam.h>
#include <asm/tlbflush.h>
#include "internal.h"
@@ -177,6 +178,13 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
if (start < vma->vm_start || start + size > vma->vm_end)
goto out;
+#ifdef __ARCH_FORCE_SHMLBA
+ /* Is the mapping cache-coherent? */
+ if ((pgoff ^ linear_page_index(vma, start)) &
+ ((SHMLBA-1) >> PAGE_SHIFT))
+ goto out;
+#endif
+
/* Must set VM_NONLINEAR before any pages are populated. */
if (!(vma->vm_flags & VM_NONLINEAR)) {
/*
