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Subject: Re: [PATCH V5 1/3] mm: Add get_user_pages_cma_migrate
To:     Alexey Kardashevskiy <aik@ozlabs.ru>, akpm@linux-foundation.org,
        Michal Hocko <mhocko@kernel.org>, mpe@ellerman.id.au,
        paulus@samba.org, David Gibson <david@gibson.dropbear.id.au>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org
References: <20181219034047.16305-1-aneesh.kumar@linux.ibm.com>
 <20181219034047.16305-2-aneesh.kumar@linux.ibm.com>
 <e9c9b68a-a31b-ab59-902a-73401a89f72a@ozlabs.ru>
From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Date:   Thu, 20 Dec 2018 10:52:09 +0530
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On 12/20/18 9:49 AM, Alexey Kardashevskiy wrote:
> 
> 
> On 19/12/2018 14:40, Aneesh Kumar K.V wrote:
>> This helper does a get_user_pages_fast and if it find pages in the CMA area
>> it will try to migrate them before taking page reference. This makes sure that
>> we don't keep non-movable pages (due to page reference count) in the CMA area.
>> Not able to move pages out of CMA area result in CMA allocation failures.
>>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>

.....
>> +		 * We did migrate all the pages, Try to get the page references again
>> +		 * migrating any new CMA pages which we failed to isolate earlier.
>> +		 */
>> +		drain_allow = true;
>> +		goto get_user_again;
> 
> 
> So it is possible to have pages pinned, then successfully migrated
> (migrate_pages() returned 0), then pinned again, then some pages may end
> up in CMA again and migrate again and nothing seems to prevent this loop
> from being endless. What do I miss?
> 

pages used as target page for migration won't be allocated from CMA region.

-aneesh