linux-kernel - Re: get_user_pages() on an mmap()ed file allowed? What to do if 0 < get_user_pages(..., nr_pages, ...) < nr

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Date:	Tue, 4 Aug 2009 18:50:58 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Leon Woestenberg <leon.woestenberg@...il.com>
Cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: get_user_pages() on an mmap()ed file allowed? What to do if 0 <
  get_user_pages(..., nr_pages, ...) < nr_pages?

On Tue, 4 Aug 2009 10:57:33 +0200
Leon Woestenberg <leon.woestenberg@...il.com> wrote:


> >> - what should a driver do when get_user_pages() returns less pages
> >> than requested?
> >
> > Probably put_page the pages gotten then report the surprise;
> > perhaps, before putting the pages gotten, try get_user_pages
> > on the next alone, to see what error code is returned for that.
> >
> > Unless it's happy to work with fewer pages than requested,
> > in which case work with them and ignore the surprise.
> >
> I expect a certain amount of data to be DMA'd from the PCI device to
> the file mmap, so I'ld rather map the complete file before I start
> DMA.
> 
I wonder.... If your device does DMA from-PCI-to-user, then,

  rc = get_user_pages(current, current->mm, start & PAGE_MASK,
nr_pages, 0 /* do not write*/, 1 /* do force */, pages, NULL);

This is *write* access. isn't it ? (contents of pages got by this call
will be overwritten by DMA ?) 

About (rc != nr_pages) case, I doubt there are difference between
mmap region (or size of file) and [start, start+count) passed to you device.

Thanks,
-Kame


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