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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Andy Grover <andy.grover@oracle.com>
Subject: Re: [PATCH 1/1] Document get_user_pages_fast()
Cc: kosaki.motohiro@jp.fujitsu.com, linux-kernel@vger.kernel.org,
       mingo@redhat.com, npiggin@suse.de, akpm@linux-foundation.org
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Date: Fri, 10 Apr 2009 11:17:22 +0900 (JST)
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> +/**
> + * 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 and Cow change owner process.

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