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

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Date:   Thu, 20 Dec 2018 17:20:15 +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 16:52, Aneesh Kumar K.V wrote:
> 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.

isolate==migrate?

If we failed to migrate, then migrate_pages() returns non zero (positive
or negative), we set migrate_allow to false, empty the cma_page_list
and repeat but we won't add anything to cma_page_list as
migrate_allow==false.

If we succeeded to migrate, then we repeat the loop with
migrate_allow==true but it does not matter as is_migrate_cma_page() is
expected to return false because we just successfully migrated
_everything_ so we won't be adding anything to cma_page_list either.

What have I missed?

-- 
Alexey