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

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Date:	Wed, 30 Jul 2008 10:29:28 -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 Wed, 30 Jul 2008 20:23:01 +0300 Avi Kivity <avi@...ranet.com> wrote:

> Andrew Morton wrote:
> >> o other arch 
> >> would define CONFIG_HAVE_GET_USER_PAGES_FAST.
> >>
> >>     
> >
> > That's presently true in Linus mainline.  The powerpc implementation is
> > in -mm, but apprarently needs a bit more work.
> 
> Wouldn't a generic get_user_pages_fast() implementation which calls 
> get_user_pages_slow() if the arch hadn't implemented the former reduce a 
> lot of #ifdefs in call sites?
> 

That is how it is presently implemented.

#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

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