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

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Date:   Thu, 20 Dec 2018 11:22:32 +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 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@...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.
> 
> 
> 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