Re: [RFC PATCH] vhost, mm: make sure that oom_reaper doesn't reap memory read by vhost

On Sun 19-06-16 23:35:43, Michal Hocko wrote:

On Sat 18-06-16 03:09:02, Michael S. Tsirkin wrote:

quoted

On Fri, Jun 17, 2016 at 11:00:17AM +0200, Michal Hocko wrote:

[...]

quoted

quoted

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 349557825428..b1f314fca3c8 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h

@@ -76,6 +76,28 @@ static inline unsigned long __copy_from_user_nocache(void *to,
 #endif		/* ARCH_HAS_NOCACHE_UACCESS */
 
 /*
+ * A safe variant of __get_user for for use_mm() users to have a
+ * gurantee that the address space wasn't reaped in the background
+ */
+#define __get_user_mm(mm, x, ptr)				\
+({								\
+	int ___gu_err = __get_user(x, ptr);			\
+	if (!___gu_err && test_bit(MMF_UNSTABLE, &mm->flags))	\

test_bit is somewhat expensive. See my old mail
	x86/bitops: implement __test_bit

Do you have a msg_id?

quoted

I dropped it as virtio just switched to simple &/| for features,
but we might need something like this now.

Is this such a hot path that something like this would make a visible
difference? 

OK, so I've tried to apply your patch [1] and updated both __get_user_mm
and __copy_from_user_mm and the result is a code size reduction:
   text    data     bss     dec     hex filename
  12835       2      32   12869    3245 drivers/vhost/vhost.o
  12882       2      32   12916    3274 drivers/vhost/vhost.o.before

This is really tiny and I cannot tell anything about the performance. Should
I resurrect your patch and push it together with this change or this can happen
later?

[1] http://lkml.kernel.org/r/1440776707-22016-1-git-send-email-mst@redhat.com
-- 
Michal Hocko
SUSE Labs