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 [thread overview]
Message-ID: <6297AD92-8D0E-4BEC-8E1F-5C5AC32FA128@nvidia.com> (raw)
In-Reply-To: <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 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@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 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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next prev parent reply other threads:[~2021-04-14 15:04 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-13 21:24 [v2 RFC PATCH 0/7] mm: thp: use generic THP migration for NUMA hinting fault Yang Shi
2021-04-13 21:24 ` [v2 PATCH 1/7] mm: memory: add orig_pmd to struct vm_fault Yang Shi
2021-05-17 15:09 ` Mel Gorman
2021-05-17 19:39 ` Yang Shi
2021-05-18 7:36 ` Mel Gorman
2021-05-18 17:03 ` Yang Shi
2021-04-13 21:24 ` [v2 PATCH 2/7] mm: memory: make numa_migrate_prep() non-static Yang Shi
2021-05-17 15:11 ` Mel Gorman
2021-04-13 21:24 ` [v2 PATCH 3/7] mm: thp: refactor NUMA fault handling Yang Shi
2021-04-14 2:43 ` Huang, Ying
2021-04-14 17:15 ` Yang Shi
2021-05-17 15:27 ` Mel Gorman
2021-05-17 19:41 ` Yang Shi
2021-04-13 21:24 ` [v2 PATCH 4/7] mm: migrate: account THP NUMA migration counters correctly Yang Shi
2021-05-17 15:28 ` Mel Gorman
2021-04-13 21:24 ` [v2 PATCH 5/7] mm: migrate: don't split THP for misplaced NUMA page Yang Shi
2021-05-17 15:29 ` Mel Gorman
2021-04-13 21:24 ` [v2 PATCH 6/7] mm: migrate: check mapcount for THP instead of ref count Yang Shi
2021-04-14 3:00 ` Huang, Ying
2021-04-14 15:02 ` Zi Yan [this message]
2021-04-15 6:45 ` Huang, Ying
2021-04-15 18:57 ` Zi Yan
2021-04-14 17:23 ` Yang Shi
2021-04-13 21:24 ` [v2 PATCH 7/7] mm: thp: skip make PMD PROT_NONE if THP migration is not supported Yang Shi
2021-05-17 15:30 ` Mel Gorman
2021-05-03 21:58 ` [v2 RFC PATCH 0/7] mm: thp: use generic THP migration for NUMA hinting fault Yang Shi
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