linux-kernel - Re: [PATCH V5 1/3] mm: Add get_user_pages_cma

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Message-Id: <b316eb1c-36dc-49e0-f46f-e610f29b6058@linux.ibm.com>
Date:   Thu, 20 Dec 2018 10:52:09 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>, akpm@...ux-foundation.org,
        Michal Hocko <mhocko@...nel.org>, mpe@...erman.id.au,
        paulus@...ba.org, David Gibson <david@...son.dropbear.id.au>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V5 1/3] mm: Add get_user_pages_cma_migrate

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@...ux.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