linux-kernel - Re: PFs on pages pinned with get_user

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Message-id: <200901291408.02454.frank.mehnert@sun.com>
Date:	Thu, 29 Jan 2009 14:08:02 +0100
From:	Frank Mehnert <Frank.Mehnert@....COM>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PFs on pages pinned with get_user_pages()

Peter,

On Thursday 29 January 2009, Peter Zijlstra wrote:
> On Thu, 2009-01-29 at 09:05 +0100, Frank Mehnert wrote:
> > please could someone explain me under which circumstances a pagefault,
> > either generated from kernel code or from userland code, can occur on
> > pages which are pinned with get_user_pages()?
> >
> > So far my understanding was that this can _never_ happen but I seems to
> > be wrong. Under high memory pressure I get PFs on such pages raised from
> > kernel code and the PFs are handled by do_swap_page(). When this happens,
> > page_count is 3 but page_mapped() returns false.
>
> Under memory pressure the page reclaim will first unmap the physical
> page from the virtual address range, and then try to free it.

Which means the page table entry is removed but the physical page
is not swapped out, right?

> Obviously the freeing bit fails if you hold a reference to it, but the
> unmap will work.

Right.

> After that, userspace will have to (minor) fault the stuff back in.

So do_swap_page does only 'restore' the page table entry, no further
reading from the swapfile is necessary?

> Also, that same page-reclaim, or pdflush might decide to write out dirty
> data, which will also result in (minor) faults when userspace will
> re-dirty the pages.
>
> Having a page reference will only avoid the physical page from getting
> removed from its current mapping (and thereby also pins the mapping).

Question: Is it possible to prevent these minor page faults at all?

Thank you very much for your answer!

Frank
-- 
Dr.-Ing. Frank Mehnert    Sun Microsystems    http://www.sun.com/

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