ARM: 9480/1: entry: expand comment in __switch_to

As per discussion between the developers in the mail thread
linked, expand the comment in __switch_to so that readers
of the code understand what is going on.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index ef6a657..b7faefe 100644 (file)
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -557,9 +557,16 @@ ENTRY(__switch_to)
        ldmia   r4, {r4 - sl, fp, ip, lr}       @ Load all regs saved previously
 #ifdef CONFIG_VMAP_STACK
        @
-       @ Do a dummy read from the new stack while running from the old one so
-       @ that we can rely on do_translation_fault() to fix up any stale PMD
-       @ entries covering the vmalloc region.
+       @ For a non-lazy mm switch, check_vmalloc_seq() has ensured that
+       @ that the active mm's page tables have mappings for the prev
+       @ task's stack and the next task's stack.
+       @
+       @ For a lazy mm switch the active mm's page tables have mappings
+       @ for the prev task's stack but might not have mappings for the
+       @ new task's stack. Do a dummy read from the new stack while
+       @ running from the old stack so that we can rely on
+       @ do_translation_fault() to populate missing PMD entries covering the
+       @ new task's stack in the old task's page tables.
        @
        ldr     r2, [ip]
 #ifdef CONFIG_KASAN_VMALLOC