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Date:   Thu, 05 Apr 2018 15:09:14 -0400
From:   "Zi Yan" <zi.yan@...t.com>
To:     "Michal Hocko" <mhocko@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        "Naoya Horiguchi" <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Vlastimil Babka" <vbabka@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm: consider non-anonymous thp as unmovable page

On 5 Apr 2018, at 15:04, Michal Hocko wrote:

> On Thu 05-04-18 13:58:43, Zi Yan wrote:
>> On 5 Apr 2018, at 12:03, Michal Hocko wrote:
>>
>>> On Thu 05-04-18 18:55:51, Kirill A. Shutemov wrote:
>>>> On Thu, Apr 05, 2018 at 05:05:47PM +0200, Michal Hocko wrote:
>>>>> On Thu 05-04-18 16:40:45, Kirill A. Shutemov wrote:
>>>>>> On Thu, Apr 05, 2018 at 02:48:30PM +0200, Michal Hocko wrote:
>>>>> [...]
>>>>>>> RIght, I confused the two. What is the proper layer to fix that then?
>>>>>>> rmap_walk_file?
>>>>>>
>>>>>> Maybe something like this? Totally untested.
>>>>>
>>>>> This looks way too complex. Why cannot we simply split THP page cache
>>>>> during migration?
>>>>
>>>> This way we unify the codepath for archictures that don't support THP
>>>> migration and shmem THP.
>>>
>>> But why? There shouldn't be really nothing to prevent THP (anon or
>>> shemem) to be migratable. If we cannot migrate it at once we can always
>>> split it. So why should we add another thp specific handling all over
>>> the place?
>>
>> Then, it would be much easier if your "unclutter thp migration" patches is merged,
>> plus the patch below:
>
> Good point. Except I would prefer a less convoluted condition
>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 60531108021a..b4087aa890f5 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1138,7 +1138,9 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>>         int rc = MIGRATEPAGE_SUCCESS;
>>         struct page *newpage;
>>
>> -       if (!thp_migration_supported() && PageTransHuge(page))
>> +       if ((!thp_migration_supported() ||
>> +            (thp_migration_supported() && !PageAnon(page))) &&
>> +           PageTransHuge(page))
>>                 return -ENOMEM;
>
> What about this?
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 5d0dc7b85f90..cd02e2bdf37c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1138,7 +1138,11 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  	int rc = MIGRATEPAGE_SUCCESS;
>  	struct page *newpage;
>
> -	if (!thp_migration_supported() && PageTransHuge(page))
> +	/*
> +	 * THP pagecache or generally non-migrateable THP need to be split
> +	 * up before migration
> +	 */
> +	if (PageTransHuge(page) && (!thp_migration_supported() || !PageAnon(page)))
>  		return -ENOMEM;
>
>  	newpage = get_new_page(page, private);

I think it works and is better than mine.

Reviewed-by: Zi Yan <zi.yan@...rutgers.edu>

--
Best Regards
Yan Zi

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