follow_page() performance regressions

[Paul Mundt]

(adding Linus and l-k to Cc..)

On Tue, Jul 01, 2008 at 01:24:39PM +0900, Hideo Saito wrote:
> I have a question as to performance of kernel on linux-2.6.25.9, but
> this may be off-topic, because the problem is not related to the SH
> architecture directly.
> 
> When I tested Hackbench benchmark test on our platform(SH7780, 400MHz),
> the performance decreases as follows.
> 
> The result on linux-2.6.25.9:
> #tcsh> ./hackbench 40 ; time ./hackbench 40 ; time ./hackbench 40
> Time: 69.858
> Time: 69.810
> Time: 70.356
> 
> The result on linux-2.6.25.8:
> #tcsh> ./hackbench 40 ; time ./hackbench 40 ; time ./hackbench 40
> Time: 65.353
> Time: 65.622
> Time: 65.413
> 
> I confirmed that the cause is in the following changes. Isn't this
> decrease of performance avoided, though it seems that this code is
> changed in order to handle ZERO_PAGE?
> 
> --- a/mm/memory.c
> +++ b/mm/memory.c
>     pte = *ptep;
>     if (!pte_present(pte))
> -       goto unlock;
> +       goto no_page;
>     if ((flags & FOLL_WRITE) && !pte_write(pte))
>         goto unlock;
>     page = vm_normal_page(vma, address, pte);
>     if (unlikely(!page))
> -       goto unlock;
> +       goto bad_page;
> 
>     if (flags & FOLL_GET)
>         get_page(page);

git blame points to:

commit 89f5b7da2a6bad2e84670422ab8192382a5aeb9f
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date:   Fri Jun 20 11:18:25 2008 -0700

    Reinstate ZERO_PAGE optimization in 'get_user_pages()' and fix XIP

...

So yes, the ZERO_PAGE handling in follow_page() is causing the slow-down here.
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