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

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Message-ID: <20131211170844.GA21700@redhat.com>
Date:	Wed, 11 Dec 2013 18:08:44 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 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?


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 ?

Oleg.

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