linux-kernel - Re: [PATCH 10/23] userfaultfd: add new syscall to provide memory externalization

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Message-ID: <CA+55aFyPY9PtGLaK=TxBX_bU44MBCe53yLXDFzU43zJqbwzQ6Q@mail.gmail.com>
Date:	Fri, 15 May 2015 11:22:09 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, qemu-devel@...gnu.org,
	KVM list <kvm@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Sanidhya Kashyap <sanidhya.gatech@...il.com>,
	zhang.zhanghailiang@...wei.com,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Feiner <pfeiner@...gle.com>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>
Subject: Re: [PATCH 10/23] userfaultfd: add new syscall to provide memory externalization

On Fri, May 15, 2015 at 9:04 AM, Andrea Arcangeli <aarcange@...hat.com> wrote:
>
> To fix it I added this along a comment:

Ok, this looks good as a explanation/fix for the races (and also as an
example of my worry about waitqueue_active() use in general).

However, it now makes me suspect that the optimistic "let's check if
they are even active" may not be worth it any more. You're adding a
"smp_mb()" in order to avoid taking the real lock. Although I guess
there are two locks there (one for each wait-queue) so maybe it's
worth it.

                    Linus
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