linux-kernel - Re: [PATCH] mm: introduce MADV_CLR

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Message-ID: <20170530074408.GA7969@dhcp22.suse.cz>
Date:   Tue, 30 May 2017 09:44:08 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

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