linux-kernel - Re: [PATCH V4 0/3] * mm/kvm/vfio/ppc64: Migrate compound pages out of CMA region

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Date:   Fri, 7 Dec 2018 15:12:26 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Alexey Kardashevskiy <aik@...abs.ru>, mpe@...erman.id.au,
        paulus@...ba.org, David Gibson <david@...son.dropbear.id.au>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V4 0/3] * mm/kvm/vfio/ppc64: Migrate compound pages out
 of CMA region

On Wed, 21 Nov 2018 14:52:56 +0530 "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> wrote:

> Subject: [PATCH V4 0/3] * mm/kvm/vfio/ppc64: Migrate compound pages out of CMA region

Asterisk in title is strange?

> ppc64 use CMA area for the allocation of guest page table (hash page table). We won't
> be able to start guest if we fail to allocate hash page table. We have observed
> hash table allocation failure because we failed to migrate pages out of CMA region
> because they were pinned. This happen when we are using VFIO. VFIO on ppc64 pins
> the entire guest RAM. If the guest RAM pages get allocated out of CMA region, we
> won't be able to migrate those pages. The pages are also pinned for the lifetime of the
> guest.
> 
> Currently we support migration of non-compound pages. With THP and with the addition of
>  hugetlb migration we can end up allocating compound pages from CMA region. This
> patch series add support for migrating compound pages. The first path adds the helper
> get_user_pages_cma_migrate() which pin the page making sure we migrate them out of
> CMA region before incrementing the reference count. 

Very little review activity.  Perhaps Andrey and/or Michal can find the
time..

> mm/migrate.c            | 108 ++++++++++++++++++++++++++++++++++++++++

can we make this code disappear when CONFIG_CMA=n?