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Date: Thu, 22 Oct 2015 17:30:55 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 14/23] userfaultfd: wake pending userfaults
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To: Peter Zijlstra <peterz@infradead.org>
Cc: kvm@vger.kernel.org, qemu-devel@nongnu.org, Sanidhya Kashyap <sanidhya.gatech@gmail.com>, Dave Hansen <dave.hansen@intel.com>, zhang.zhanghailiang@huawei.com, Pavel Emelyanov <xemul@parallels.com>, Hugh Dickins <hughd@google.com>, Mel Gorman <mgorman@suse.de>, "Huangpeng (Peter)" <peter.huangpeng@huawei.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Andres Lagar-Cavilla <andreslc@google.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, linux-mm@kvack.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>, Johannes Weiner <hannes@cmpxchg.org>, Paolo Bonzini <pbonzini@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, Peter Feiner <pfeiner@google.com>

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.