Re: [v2 PATCH 6/7] mm: migrate: check mapcount for THP instead of ref count

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, gerald.schaefer@linux.ibm.com,
	hca@linux.ibm.com, gor@linux.ibm.com, borntraeger@de.ibm.com,
	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	[thread overview]
Message-ID: <F357B0CC-B7A4-4140-9009-BD0A5B551F0F@nvidia.com> (raw)
In-Reply-To: <87fszsoxjx.fsf@yhuang6-desk1.ccr.corp.intel.com>

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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 refcount 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 behavior.
>>>
>>> What prevents us from migrating shared THP?  If no, why not just remove
>>> 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. Unless 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 worth 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.

—
Best Regards,
Yan Zi

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