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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>
 <b316eb1c-36dc-49e0-f46f-e610f29b6058@linux.ibm.com>
 <23cae5a6-4370-224c-523c-ab6ee940cf87@ozlabs.ru>
From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Date:   Thu, 20 Dec 2018 11:22:32 +0530
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On 12/20/18 11:18 AM, Alexey Kardashevskiy wrote:
> 
> 
> On 20/12/2018 16:22, Aneesh Kumar K.V wrote:
>> 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.
> 
> 
> Then migrate_allow should be set to "false" regardless what
> migrate_pages() returned and then I am totally missing the point of this
> goto and going through the loop again even when we know for sure it
> won't do literally anything but checking is_migrate_cma_page() even
> though we know pages won't be allocated from CMA.
> 

Because we might have failed to isolate all the pages in the first attempt.

-aneesh