linux-kernel - Re: [PATCH 14/23] userfaultfd: wake pending userfaults

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Message-ID: <20151022153055.GC1331@redhat.com>
Date:	Thu, 22 Oct 2015 17:30:55 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	qemu-devel@...gnu.org, kvm@...r.kernel.org,
	linux-api@...r.kernel.org, Pavel Emelyanov <xemul@...allels.com>,
	Sanidhya Kashyap <sanidhya.gatech@...il.com>,
	zhang.zhanghailiang@...wei.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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 14/23] userfaultfd: wake pending userfaults

On Thu, Oct 22, 2015 at 05:15:09PM +0200, Peter Zijlstra wrote:
> Indefinitely is such a long time, we should try and finish
> computation before the computer dies etc. :-)

Indefinitely as read_seqcount_retry, eventually it makes progress.

Even returning 0 from the page fault can trigger it again
indefinitely, so VM_FAULT_RETRY isn't fundamentally different from
returning 0 and retrying the page fault again later. So it's not clear
why VM_FAULT_RETRY can only try once more.

FAULT_FLAG_TRIED as a message to the VM so it starts to do heavy
locking and block more aggressively is actually useful as such, but it
shouldn't be a replacement of FAULT_FLAG_ALLOW_RETRY. What I meant
with removing FAULT_FLAG_TRIED is really about converting it to an
hint, but not controlling if the page fault can keep retrying
in-kernel.
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