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

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Message-Id: <7a14631d-8077-e20f-d8a9-740710406168@linux.ibm.com>
Date:   Sat, 8 Dec 2018 22:34:39 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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 12/8/18 4:42 AM, Andrew Morton wrote:
> 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?

My mistake while editing git-format-patch cover-letter.

> 
>> 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?
> 


We can definitely do

static inline int get_user_pages_cma_migrate(unsigned long start, int 
nr_pages, int write,  struct page **pages)
{
	
	return get_user_pages_fast(start, nr_pages, write, pages);
}

with #ifdef CONFIG_CMA around but that is unnecessary #ifdef in the 
code. If CMA config is disabled, we will not be doing any migrate. Hence 
wondering whether we need an alternative definition for CONFIG_CMA=n

-aneesh