Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        David Gibson <david@gibson.dropbear.id.au>,
        Andrea Arcangeli <aarcange@redhat.com>, mpe@ellerman.id.au,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH V6 0/4] mm/kvm/vfio/ppc64: Migrate compound pages out of CMA region
In-Reply-To: <20190108115620.6ec22e7d60b86d5f609d5a87@linux-foundation.org>
References: <20190108045110.28597-1-aneesh.kumar@linux.ibm.com> <20190108115620.6ec22e7d60b86d5f609d5a87@linux-foundation.org>
Date:   Wed, 09 Jan 2019 14:11:25 +0530
MIME-Version: 1.0
Content-Type: text/plain
Message-Id: <875zuyjk96.fsf@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Andrew Morton <akpm@linux-foundation.org> writes:

> On Tue,  8 Jan 2019 10:21:06 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:
>
>> 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. 
>
> Does this code do anything for architectures other than powerpc?  If
> not, should we be adding the ifdefs to avoid burdening other
> architectures with unused code?

Any architecture enabling CMA may need this. I will move most of this below
CONFIG_CMA.

-aneesh