Performance regression in write() syscall

[sqazi@google.com (Salman Qazi)]

While the introduction of __copy_from_user_nocache (see commit:
0812a579c92fefa57506821fa08e90f47cb6dbdd) may have been an improvement
for sufficiently large writes, there is evidence to show that it is
deterimental for small writes.  Unixbench's fstime test gives the
following results for 256 byte writes with MAX_BLOCK of 2000:
    
    2.6.29-rc6 ( 5 samples, each in KB/sec ):
    283750, 295200, 294500, 293000, 293300
    
    2.6.29-rc6 + this patch (5 samples, each in KB/sec):
    313050, 3106750, 293350, 306300, 307900

    2.6.18
    395700, 342000, 399100, 366050, 359850

    See w_test() in src/fstime.c in unixbench version 4.1.0.  Basically, the above test
    consists of counting how much we can write in this manner:

    alarm(10);
    while (!sigalarm) {
            for (f_blocks = 0; f_blocks < 2000; ++f_blocks) {
                   write(f, buf, 256);
            }
            lseek(f, 0L, 0);
    }

I realised that there are other components to the write syscall regression
that are not addressed here.  I will send another email shortly stating the
source of another one.

Signed-off-by: Salman Qazi <sqazi@google.com>
---
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index 84210c4..efe7315 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -192,14 +192,20 @@ static inline int __copy_from_user_nocache(void *dst, const void __user *src,
 					   unsigned size)
 {
 	might_sleep();
-	return __copy_user_nocache(dst, src, size, 1);
+	if (likely(size >= PAGE_SIZE))
+		return __copy_user_nocache(dst, src, size, 1);
+	else
+		return __copy_from_user(dst, src, size);
 }
 
 static inline int __copy_from_user_inatomic_nocache(void *dst,
 						    const void __user *src,
 						    unsigned size)
 {
-	return __copy_user_nocache(dst, src, size, 0);
+	if (likely(size >= PAGE_SIZE))
+		return __copy_user_nocache(dst, src, size, 0);
+	else
+		return __copy_from_user_inatomic(dst, src, size);
 }
 
 unsigned long