From 799c10559d60f159ab2232203f222f18fa3c4a5f Mon Sep 17 00:00:00 2001

From: Linus Torvalds <torvalds@linux-foundation.org>

Date: Fri, 15 Oct 2010 11:09:28 -0700

Subject: [PATCH] De-pessimize rds_page_copy_user

Don't try to "optimize" rds_page_copy_user() by using kmap_atomic() and

the unsafe atomic user mode accessor functions.  It's actually slower

than the straightforward code on any reasonable modern CPU.

Back when the code was written (although probably not by the time it was

actually merged, though), 32-bit x86 may have been the dominant

architecture.  And there kmap_atomic() can be a lot faster than kmap()

(unless you have very good locality, in which case the virtual address

caching by kmap() can overcome all the downsides).

But these days, x86-64 may not be more populous, but it's getting there

(and if you care about performance, it's definitely already there -

you'd have upgraded your CPU's already in the last few years).  And on

x86-64, the non-kmap_atomic() version is faster, simply because the code

is simpler and doesn't have the "re-try page fault" case.

People with old hardware are not likely to care about RDS anyway, and

the optimization for the 32-bit case is simply buggy, since it doesn't

verify the user addresses properly.

Reported-by: Dan Rosenberg <drosenberg@vsecurity.com>

Acked-by: Andrew Morton <akpm@linux-foundation.org>

Cc: stable@kernel.org

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

---

 net/rds/page.c |   27 +++++++--------------------

 1 files changed, 7 insertions(+), 20 deletions(-)

diff --git a/net/rds/page.c b/net/rds/page.c

index 595a952..1dfbfea 100644

--- a/net/rds/page.c

+++ b/net/rds/page.c

@@ -57,30 +57,17 @@ int rds_page_copy_user(struct page *page, unsigned long offset,

 	unsigned long ret;

 	void *addr;

-	if (to_user)

+	addr = kmap(page);

+	if (to_user) {

 		rds_stats_add(s_copy_to_user, bytes);

-	else

+		ret = copy_to_user(ptr, addr + offset, bytes);

+	} else {

 		rds_stats_add(s_copy_from_user, bytes);

-

-	addr = kmap_atomic(page, KM_USER0);

-	if (to_user)

-		ret = __copy_to_user_inatomic(ptr, addr + offset, bytes);

-	else

-		ret = __copy_from_user_inatomic(addr + offset, ptr, bytes);

-	kunmap_atomic(addr, KM_USER0);

-

-	if (ret) {

-		addr = kmap(page);

-		if (to_user)

-			ret = copy_to_user(ptr, addr + offset, bytes);

-		else

-			ret = copy_from_user(addr + offset, ptr, bytes);

-		kunmap(page);

-		if (ret)

-			return -EFAULT;

+		ret = copy_from_user(addr + offset, ptr, bytes);

 	}

+	kunmap(page);

-	return 0;

+	return ret ? -EFAULT : 0;

 }

 EXPORT_SYMBOL_GPL(rds_page_copy_user);

-- 

1.7.3.2