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From:   "Zi Yan" <ziy@nvidia.com>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     "Yang Shi" <shy828301@gmail.com>, mgorman@suse.de,
        kirill.shutemov@linux.intel.com, mhocko@suse.com, hughd@google.com,
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        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [v2 PATCH 6/7] mm: migrate: check mapcount for THP instead of ref
 count
Date:   Thu, 15 Apr 2021 14:57:48 -0400
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On 15 Apr 2021, at 2:45, Huang, Ying wrote:

> "Zi Yan" <ziy@nvidia.com> writes:
>
>> On 13 Apr 2021, at 23:00, Huang, Ying wrote:
>>
>>> Yang Shi <shy828301@gmail.com> writes:
>>>
>>>> The generic migration path will check refcount, so no need check ref=
count here.
>>>> But the old code actually prevents from migrating shared THP (mapped=
 by multiple
>>>> processes), so bail out early if mapcount is > 1 to keep the behavio=
r.
>>>
>>> What prevents us from migrating shared THP?  If no, why not just remo=
ve
>>> the old refcount checking?
>>
>> If two or more processes are in different NUMA nodes, a THP shared by =
them can be
>> migrated back and forth between NUMA nodes, which is quite costly. Unl=
ess we have
>> a better way of figuring out a good location for such pages to reduce =
the number
>> of migration, it might be better not to move them, right?
>>
>
> Some mechanism has been provided in should_numa_migrate_memory() to
> identify the shared pages from the private pages.  Do you find it
> doesn't work well in some situations?
>
> The multiple threads in one process which run on different NUMA nodes
> may share pages too.  So it isn't a good solution to exclude pages
> shared by multiple processes.

After recheck the patch, it seems that no shared THP migration here is a =
side effect
of the original page_count check, which might not be intended and be wort=
h fixing.
But Yang just want to solve one problem, simplifying THP NUMA migration,
at a time. Maybe a separate patch would be better for both discussing and=
 fixing this problem.


=E2=80=94
Best Regards,
Yan Zi

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