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

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Message-ID: <CA+55aFw_emAUOr5BYx0TfTCGEOrW6-ZRATWbm00DvCXsGD7=DQ@mail.gmail.com>
Date:	Tue, 10 Dec 2013 14:33:16 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Tue, Dec 10, 2013 at 2:19 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Shouldn't we do something like the attached?

So I think that kernel/futex.c part of the patch might be a good idea,
but on x86-64 (which is what Dave is running), the

        if (end >> __VIRTUAL_MASK_SHIFT)

test in get_user_pages_fast() should have been equivalent. And even on
32-bit, we do check the _PAGE_USER bits in the page tables, so I guess
it's all good on a get_user_pages_fast() side.

So never mind. It's not the address checking.

And I think I see what's up.

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).

Comments?

                Linus

View attachment "patch.diff" of type "text/plain" (1255 bytes)