linux-kernel - Re: [PATCH 1/2] Export get_user_pages

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Message-Id: <20080731000257.59830c25.akpm@linux-foundation.org>
Date:	Thu, 31 Jul 2008 00:02:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Adrian Bunk <bunk@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lguest <lguest@...abs.org>, linux-kernel@...r.kernel.org,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [PATCH 1/2] Export get_user_pages_fast

On Thu, 31 Jul 2008 09:43:59 +0300 Avi Kivity <avi@...ranet.com> wrote:

> > #ifdef CONFIG_HAVE_GET_USER_PAGES_FAST
> > /*
> >  * get_user_pages_fast provides equivalent functionality to get_user_pages,
> >  * operating on current and current->mm (force=0 and doesn't return any vmas).
> >  *
> >  * get_user_pages_fast may take mmap_sem and page tables, so no assumptions
> >  * can be made about locking. get_user_pages_fast is to be implemented in a
> >  * way that is advantageous (vs get_user_pages()) when the user memory area is
> >  * already faulted in and present in ptes. However if the pages have to be
> >  * faulted in, it may turn out to be slightly slower).
> >  */
> > int get_user_pages_fast(unsigned long start, int nr_pages, int write,
> > 			struct page **pages);
> >
> > #else
> > /*
> >  * Should probably be moved to asm-generic, and architectures can include it if
> >  * they don't implement their own get_user_pages_fast.
> >  */
> > #define get_user_pages_fast(start, nr_pages, write, pages)	\
> > ({								\
> > 	struct mm_struct *mm = current->mm;			\
> > 	int ret;						\
> > 								\
> > 	down_read(&mm->mmap_sem);				\
> > 	ret = get_user_pages(current, mm, start, nr_pages,	\
> > 					write, 0, pages, NULL);	\
> > 	up_read(&mm->mmap_sem);					\
> > 								\
> > 	ret;							\
> > })
> > #endif
> >
> >   
> 
> Surprised.  Why not an out-of-line function?  Would eliminate the 
> present discussion as well.

Excellent question!
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