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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,
        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:   Wed, 14 Apr 2021 11:02:58 -0400
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In-Reply-To: <87k0p5sh7h.fsf@yhuang6-desk1.ccr.corp.intel.com>
References: <20210413212416.3273-1-shy828301@gmail.com>
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 <87k0p5sh7h.fsf@yhuang6-desk1.ccr.corp.intel.com>
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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 refco=
unt here.
>> But the old code actually prevents from migrating shared THP (mapped b=
y 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 the=
m 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?

>
> Best Regards,
> Huang, Ying
>
>> Signed-off-by: Yang Shi <shy828301@gmail.com>
>> ---
>>  mm/migrate.c | 16 ++++------------
>>  1 file changed, 4 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index a72994c68ec6..dc7cc7f3a124 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -2067,6 +2067,10 @@ static int numamigrate_isolate_page(pg_data_t *=
pgdat, struct page *page)
>>
>>  	VM_BUG_ON_PAGE(compound_order(page) && !PageTransHuge(page), page);
>>
>> +	/* Do not migrate THP mapped by multiple processes */
>> +	if (PageTransHuge(page) && page_mapcount(page) > 1)
>> +		return 0;
>> +
>>  	/* Avoid migrating to a node that is nearly full */
>>  	if (!migrate_balanced_pgdat(pgdat, compound_nr(page)))
>>  		return 0;
>> @@ -2074,18 +2078,6 @@ static int numamigrate_isolate_page(pg_data_t *=
pgdat, struct page *page)
>>  	if (isolate_lru_page(page))
>>  		return 0;
>>
>> -	/*
>> -	 * migrate_misplaced_transhuge_page() skips page migration's usual
>> -	 * check on page_count(), so we must do it here, now that the page
>> -	 * has been isolated: a GUP pin, or any other pin, prevents migratio=
n.
>> -	 * The expected page count is 3: 1 for page's mapcount and 1 for the=

>> -	 * caller's pin and 1 for the reference taken by isolate_lru_page().=

>> -	 */
>> -	if (PageTransHuge(page) && page_count(page) !=3D 3) {
>> -		putback_lru_page(page);
>> -		return 0;
>> -	}
>> -
>>  	page_lru =3D page_is_file_lru(page);
>>  	mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON + page_lru,
>>  				thp_nr_pages(page));


=E2=80=94
Best Regards,
Yan Zi

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