linux-kernel - Re: process 'stuck' at exit.

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Date:	Wed, 11 Dec 2013 18:18:07 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: process 'stuck' at exit.

On Wed, 11 Dec 2013, Oleg Nesterov wrote:
> On 12/10, Linus Torvalds wrote:
> >
> > I think what happens is:
> >  - get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only)
> >  - get_user_pages_fast(address, 1, 0, &page) succeeds and gets a large-page
> >  - __get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only).
> >
> > so what triggers this is likely that Dave now does large-pages, and
> > one of them is a read-only mapping.
> >
> > So I would suggest replacing the second "1" in the
> > __get_user_pages_fast() call with a "!ro" instead. So how about this
> > second patch instead (the access_ok() move remains).
> 
> I know almost nothing about THP, but why we may need write == true in
> this case?
> 
> IOW,
> 
> > -		if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) {
> > +		if (likely(__get_user_pages_fast(address, 1, !ro, &page) == 1)) {
> 
> can't
> 		if (likely(__get_user_pages_fast(address, 1, 0, &page) == 1)) {
> 
> work?

If the futex call needs to write to that address, we need a writeable
mapping, right? And get_user_pages_fast() will verify that for us.
 
> I have to admit, I do not understand why we can't avoid this altogether.
> 
> __get_page_tail() can find the stable ->first_page, why get_futex_key()
> can't ?

Because it can be split under us.

Thanks,

	tglx
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