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

[Zi Yan]

On 13 Apr 2021, at 23:00, Huang, Ying wrote:

> Yang Shi <shy828301@xxxxxxxxx> 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?

>
> Best Regards,
> Huang, Ying
>
>> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
>> ---
>>  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 migration.
>> -	 * 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) != 3) {
>> -		putback_lru_page(page);
>> -		return 0;
>> -	}
>> -
>>  	page_lru = page_is_file_lru(page);
>>  	mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON + page_lru,
>>  				thp_nr_pages(page));


—
Best Regards,
Yan Zi

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