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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH V6 0/4] mm/kvm/vfio/ppc64: Migrate compound pages out of
 CMA region
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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