linux-kernel - Re: [PATCH] x86: Optimize tail handling for copy

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Date:	Wed, 30 Jul 2008 14:07:58 +0200
From:	Vitaly Mayatskikh <v.mayatskih@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] x86: Optimize tail handling for copy_user

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Mon, 28 Jul 2008, Vitaly Mayatskikh wrote:
>>
>> Reduce protection faults count in copy_user_handle_tail routine by
>> limiting clear length to the end of page as was suggested by Linus.
>
> No, you did it wrong.

Optimize tail handling with regarding to flags passed to handler
routine.

Signed-off-by: Vitaly Mayatskikh <v.mayatskih@...il.com>

diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index f4df6e7..d793900 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -161,23 +161,32 @@ EXPORT_SYMBOL(copy_in_user);
 /*
  * Try to copy last bytes and clear the rest if needed.
  * Since protection fault in copy_from/to_user is not a normal situation,
- * it is not necessary to optimize tail handling.
+ * it is not necessary to do low level optimization of tail handling.
  */
 unsigned long
-copy_user_handle_tail(char *to, char *from, unsigned len, unsigned zerorest)
+copy_user_handle_tail(char *dst, char *src, unsigned remainder, unsigned flags)
 {
 	char c;
-	unsigned zero_len;
+	unsigned max_copy = remainder;
 
-	for (; len; --len) {
-		if (__get_user_nocheck(c, from++, sizeof(char)))
+	/* Don't even bother trying to cross a page in user space! */
+	if (flags & DEST_IS_USERSPACE)
+		max_copy = min(max_copy, BYTES_LEFT_IN_PAGE(dst));
+	if (flags & SOURCE_IS_USERSPACE)
+		max_copy = min(max_copy, BYTES_LEFT_IN_PAGE(src));
+
+	while (max_copy--) {
+		if (__get_user(c, src))
 			break;
-		if (__put_user_nocheck(c, to++, sizeof(char)))
+		if (__put_user(c, dst))
 			break;
+		src++;
+		dst++;
+		remainder--;
 	}
 
-	for (c = 0, zero_len = len; zerorest && zero_len; --zero_len)
-		if (__put_user_nocheck(c, to++, sizeof(char)))
-			break;
-	return len;
+	if (flags & CLEAR_REMAINDER)
+		memset(dst, 0, remainder);
+
+	return remainder;
 }

-- 
wbr, Vitaly
--
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