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

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Message-Id: <20170530101921.GA25738@rapoport-lnx>
Date:   Tue, 30 May 2017 13:19:22 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
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 Tue, May 30, 2017 at 09:44:08AM +0200, Michal Hocko wrote:
> 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?

We can register only post-copy regions, but this will cause VMA
fragmentation. Now we register the entire VMA with userfaultfd, no matter
how many pages were dirtied there since the pre-dump. If we register only
post-copy regions, we will split out the VMAs for those regions.
 
> > But then we'll have to populate these regions with
> > UFFDIO_COPY which adds quite an overhead.
> 
> How big is the performance impact?

I don't have the numbers handy, but for each post-copy range it means that
instead of memcpy() we will use ioctl(UFFDIO_COPY).

> -- 
> Michal Hocko
> SUSE Labs
 
--
Sincerely yours,
Mike.