linux-kernel - Re: Performance regression in write() syscall

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Message-Id: <E1LbvDO-0001O1-Ov@be1.7eggert.dyndns.org>
Date:	Tue, 24 Feb 2009 12:12:42 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Andi Kleen <andi@...stfloor.org>, Salman Qazi <sqazi@...gle.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>
Subject: Re: Performance regression in write() syscall

Andi Kleen <andi@...stfloor.org> wrote:
> On Mon, Feb 23, 2009 at 06:03:04PM -0800, Salman Qazi wrote:

>> @@ -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);
> 
> I think you disabled it completely, the kernel never really does
> any copies larger than page size because all its internal objects
> are page sized only.
> 
> That check would need to be higher up the VFS stack (above filemap.c code)
> before the copies are split up.

It's bigger-or-equal, so this test is correct.

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