Re: [RFC PATCH] futex: replace get_user_pages() withget_user_pages_fast()

[Peter Zijlstra]

On Thu, 2011-06-23 at 08:11 -0700, Darren Hart wrote:
> 
> On 06/22/2011 11:21 PM, KOSAKI Motohiro wrote:
> > Hello,
> > 
> > Is there any reason to take mmap_sem explicitly here?
> > Should we change gup_fast() to allow NULL argument (ie for avoid get_page)?
> > 
> > ============================
> >   down_read(&mm->mmap_sem);
> >   get_user_pages(current, current->mm)
> >   up_read(&mm->mmap_sem);
> > 
> > and
> > 
> >   get_user_pages_fast()
> > 
> > make an equivalent result, And latter would be better when mamp_sem
> > highly contended case, because it can avoid to take mmap_sem if
> > the target page doesn't need a page fault.
> 
> 
> I can't speak authoritatively here, but it seems to me that
> get_user_pages_fast falls back to get_user_pages with mmap_sem anyway,
> so this seems like a reasonable optimization for the best case, with a
> minor overhead for the slow case.
> 
> Peter, am I missing something? 

Right, its much cheaper on the fast-path where the page is found present
and writable, if we have to take the fault, the extra cost in the slow
path is neglectable.

> Is there a reason you left this as is
> during your fast gup futex update?

I probably completely missed this gup() user.

The proposed change looks good to me.
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