From mboxrd@z Thu Jan  1 00:00:00 1970
From: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE
Date: Tue, 30 May 2017 09:44:08 +0200
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Cc: Vlastimil Babka <vbabka-AlSwsSmVLrQ@public.gmane.org>, "Kirill A. Shutemov" <kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>, "Kirill A. Shutemov" <kirill.shutemov-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>, Andrea Arcangeli <aarcange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Pavel Emelyanov <xemul-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>, linux-mm <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>, lkml <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Linux API <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
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On Wed 24-05-17 17:27:36, Mike Rapoport wrote:
> On Wed, May 24, 2017 at 01:18:00PM +0200, Michal Hocko wrote:
[...]
> > Why cannot khugepaged simply skip over all VMAs which have userfault
> > regions registered? This would sound like a less error prone approach to
> > me.
> 
> khugepaged does skip over VMAs which have userfault. We could register the
> regions with userfault before populating them to avoid collapses in the
> transition period.

Why cannot you register only post-copy regions and "manually" copy the
pre-copy parts?

> But then we'll have to populate these regions with
> UFFDIO_COPY which adds quite an overhead.

How big is the performance impact?
-- 
Michal Hocko
SUSE Labs