Re: [PATCH 1/1] Document get_user_pages_fast()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 10 Apr 2009 11:17:22 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> > +/**
> > + * get_user_pages_fast() - pin user pages in memory
> > + * @start:	starting user address
> > + * @nr_pages:	number of pages from start to pin
> > + * @write:	whether pages will be written to
> > + * @pages:	array that receives pointers to the pages pinned.
> > + *		Should be at least nr_pages long.
> > + *
> > + * Attempt to pin user pages in memory without taking mm->mmap_sem.
> > + * If not successful, it will fall back to taking the lock and
> > + * calling get_user_pages().
> > + *
> > + * Returns number of pages pinned. This may be fewer than the number
> > + * requested. If nr_pages is 0 or negative, returns 0. If no pages
> > + * were pinned, returns -errno.
> > + */
> >  int __attribute__((weak)) get_user_pages_fast(unsigned long start,
> >  				int nr_pages, int write, struct page **pages)
> >  {
> 
> There is one problem.
> gup and get_page() don't provide any page pinning.
> 
> get_page() only gurantee not freed. but don't gurantee to prevent page
> reclaim

is_page_cache_freeable() measures page_count()...

> and Cow change owner process.

hm.  buggy userspace? Why is that a problem from a kernel perspective?

> Yes, current all gup caller have serious bug. especially direct-io, aio, bio
> are broken.
> 
> 
> 
>