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

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Message-ID: <23cae5a6-4370-224c-523c-ab6ee940cf87@ozlabs.ru>
Date:   Thu, 20 Dec 2018 16:48:30 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        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 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.

It should be simple gup_fast() instead of goto and then we won't need
goto/migrate_allow.


-- 
Alexey