ARM: 5691/1: fix cache aliasing issues between kmap() and kmap_atomic() with highmem

Let's suppose a highmem page is kmap'd with kmap().  A pkmap entry is
used, the page mapped to it, and the virtual cache is dirtied.  Then
kunmap() is used which does virtually nothing except for decrementing a
usage count.

Then, let's suppose the _same_ page gets mapped using kmap_atomic().
It is therefore mapped onto a fixmap entry instead, which has a
different virtual address unaware of the dirty cache data for that page
sitting in the pkmap mapping.

Fortunately it is easy to know if a pkmap mapping still exists for that
page and use it directly with kmap_atomic(), thanks to kmap_high_get().

And actual testing with a printk in the added code path shows that this
condition is actually met *extremely* frequently.  Seems that we've been
quite lucky that things have worked so well with highmem so far.

Cc: stable@kernel.org
Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/highmem.c

@@ -40,11 +40,16 @@ void *kmap_atomic(struct page *page, enum km_type type)
 {
 	unsigned int idx;
 	unsigned long vaddr;
+	void *kmap;
 
 	pagefault_disable();
 	if (!PageHighMem(page))
 		return page_address(page);
 
+	kmap = kmap_high_get(page);
+	if (kmap)
+		return kmap;
+
 	idx = type + KM_TYPE_NR * smp_processor_id();
 	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
 #ifdef CONFIG_DEBUG_HIGHMEM
@@ -80,6 +85,9 @@ void kunmap_atomic(void *kvaddr, enum km_type type)
 #else
 		(void) idx;  /* to kill a warning */
 #endif
+	} else if (vaddr >= PKMAP_ADDR(0) && vaddr < PKMAP_ADDR(LAST_PKMAP)) {
+		/* this address was obtained through kmap_high_get() */
+		kunmap_high(pte_page(pkmap_page_table[PKMAP_NR(vaddr)]));
 	}
 	pagefault_enable();
 }