[PATCH] ARM/shmem: Drop page coloring align for non-VIPT CPUs

[Dmitry Safonov]

On ARMv6 CPUs with VIPT caches there are aliasing issues: if two
different cache line indexes correspond to the same physical
address, then changes made to one of the alias might be lost
or they can overwrite each other. To overcome aliasing issues,
the align for shared mappings was introduced with:

commit 4197692eef113eeb8e3e413cc70993a5e667e5b8
Author: Russell King <rmk at flint.arm.linux.org.uk>
Date:   Wed Apr 28 22:22:33 2004 +0100

    [ARM] Fix shared mmap()ings for ARM VIPT caches.

    This allows us to appropriately align shared mappings on VIPT caches
    with aliasing issues.

Which introduced 4 pages align with SHMLBA, which resulted in
unique physical address after any tag in cache (because two upper bits
corresponding to page address get unused in tags).

As this workaround is not needed by non-VIPT caches (like most armv7
CPUs which have PIPT caches), ARM mmap() code checks if cache is VIPT
aliasing for MAP_SHARED.

The problem here is in shmat() syscall:
1. if shmaddr is NULL then do_shmat() uses arch_get_unmapped_area()
   to allocate shared mapping.
2. if shmaddr is specified then do_shmat() checks that address has
   SHMLBA alignment regardless to CPU cache aliasing.

Which results on ARMv7 CPUs that shmat() with NULL shmaddr may return
non-SHMLBA aligned address (page-aligned), but shmat() with the same
address will fail.

That is not critical issue for CRIU as after shmat() with NULL address,
we can mremap() resulted shmem to restore shared memory mappings on the
same address where they were on checkpointing.
But it's still worth fixing because we can't reliably tell from
userspace if the platform has VIPT cache, and so this mremap()
workaround is done with HUGE warning that restoring application, that
uses SHMBLA-unaligned shmem on ARMv6 CPU with VIPT cache may result
in data corruptions.

I also changed SHMLBA build-time check to init-time WARN_ON(), as
it's not constant afterward.

This is resend of the patch from discussion:
http://www.spinics.net/lists/arm-kernel/msg258870.html

Cc: linux-arm-kernel at lists.infradead.org
Cc: Will Deacon <will.deacon at arm.com>
Cc: Russell King <rmk+kernel at armlinux.org.uk>
Cc: Christopher Covington <cov at codeaurora.org>
Cc: Cyrill Gorcunov <gorcunov at openvz.org>
Cc: Pavel Emelyanov <xemul at virtuozzo.com>
Suggested-by: Russell King <rmk+kernel at armlinux.org.uk>
Signed-off-by: Dmitry Safonov <dsafonov at virtuozzo.com>
---
 arch/arm/include/asm/shmparam.h | 8 ++++----
 arch/arm/mm/copypage-v6.c       | 5 +----
 2 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/arch/arm/include/asm/shmparam.h b/arch/arm/include/asm/shmparam.h
index a5223b3a9bf9..6e1a72cbdf78 100644
--- a/arch/arm/include/asm/shmparam.h
+++ b/arch/arm/include/asm/shmparam.h
@@ -1,16 +1,16 @@
 #ifndef _ASMARM_SHMPARAM_H
 #define _ASMARM_SHMPARAM_H
 
+#include <asm/cachetype.h>
+
 /*
  * This should be the size of the virtually indexed cache/ways,
  * or page size, whichever is greater since the cache aliases
  * every size/ways bytes.
  */
-#define	SHMLBA	(4 * PAGE_SIZE)		 /* attach addr a multiple of this */
+#define	SHMLBA (cache_is_vipt_aliasing() ? 4 * PAGE_SIZE : PAGE_SIZE)
 
-/*
- * Enforce SHMLBA in shmat
- */
+/* Enforce SHMLBA in shmat */
 #define __ARCH_FORCE_SHMLBA
 
 #endif /* _ASMARM_SHMPARAM_H */
diff --git a/arch/arm/mm/copypage-v6.c b/arch/arm/mm/copypage-v6.c
index 70423345da26..9b22c530e8b6 100644
--- a/arch/arm/mm/copypage-v6.c
+++ b/arch/arm/mm/copypage-v6.c
@@ -20,10 +20,6 @@
 
 #include "mm.h"
 
-#if SHMLBA > 16384
-#error FIX ME
-#endif
-
 static DEFINE_RAW_SPINLOCK(v6_lock);
 
 /*
@@ -129,6 +125,7 @@ struct cpu_user_fns v6_user_fns __initdata = {
 
 static int __init v6_userpage_init(void)
 {
+	WARN_ON(SHMLBA > 16384);
 	if (cache_is_vipt_aliasing()) {
 		cpu_user.cpu_clear_user_highpage = v6_clear_user_highpage_aliasing;
 		cpu_user.cpu_copy_user_highpage = v6_copy_user_highpage_aliasing;
-- 
2.12.2