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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Axel Rasmussen , Yuanchu Xie , Wei Xu , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , Barry Song , LKML , linux-mm@kvack.org, John Hubbard Subject: [RFC PATCH 2/2] mm/migrate: wait for folio refcount during longterm pin migration Date: Thu, 9 Apr 2026 20:23:33 -0700 Message-ID: <20260410032333.400406-3-jhubbard@nvidia.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260410032333.400406-1-jhubbard@nvidia.com> References: <20260410032333.400406-1-jhubbard@nvidia.com> X-NVConfidentiality: public Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: SJ0PR13CA0090.namprd13.prod.outlook.com (2603:10b6:a03:2c4::35) To DM3PR12MB9416.namprd12.prod.outlook.com (2603:10b6:0:4b::8) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM3PR12MB9416:EE_|MN6PR12MB8472:EE_ X-MS-Office365-Filtering-Correlation-Id: 50a36020-959c-4860-5d2c-08de96b08e49 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014|18002099003|22082099003|56012099003; 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These references are often transient (e.g., from GPU operations like cuMemset that will complete shortly). Previously, the migration code would retry up to 10 times (NR_MAX_MIGRATE_PAGES_RETRY), but this busy-retry approach failed when the transient reference holder needed more time than the retry loop provides. Fix this by waiting up to one second for the folio's refcount to drop to the expected value before retrying migration. The wait uses wait_var_event_timeout() paired with the wake_up_var() calls added to folio_put() in the previous commit. If the timeout expires, the existing retry loop continues as before. The folio_put_wakeup_key static key is enabled for the duration of migrate_pages() so that folio_put() only wakes waiters when migration is active. Signed-off-by: John Hubbard --- mm/migrate.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/mm/migrate.c b/mm/migrate.c index 2c3d489ecf51..a5d9f85aa376 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -47,6 +47,8 @@ #include #include +#include +#include #include "internal.h" #include "swap.h" @@ -1732,6 +1734,17 @@ static void migrate_folios_move(struct list_head *src_folios, *retry += 1; *thp_retry += is_thp; *nr_retry_pages += nr_pages; + /* + * For longterm pinning, wait for references + * to be released before retrying. + */ + if (reason == MR_LONGTERM_PIN) { + int expected = folio_expected_ref_count(folio) + 1; + + wait_var_event_timeout(&folio->_refcount, + folio_ref_count(folio) <= expected, + HZ); + } break; case 0: stats->nr_succeeded += nr_pages; @@ -1941,6 +1954,17 @@ static int migrate_pages_batch(struct list_head *from, retry++; thp_retry += is_thp; nr_retry_pages += nr_pages; + /* + * For longterm pinning, wait for references + * to be released. + */ + if (reason == MR_LONGTERM_PIN) { + int expected = folio_expected_ref_count(folio) + 1; + + wait_var_event_timeout(&folio->_refcount, + folio_ref_count(folio) <= expected, + HZ); + } break; case 0: list_move_tail(&folio->lru, &unmap_folios); @@ -2085,6 +2109,9 @@ int migrate_pages(struct list_head *from, new_folio_t get_new_folio, memset(&stats, 0, sizeof(stats)); + if (reason == MR_LONGTERM_PIN) + static_branch_inc(&folio_put_wakeup_key); + rc_gather = migrate_hugetlbs(from, get_new_folio, put_new_folio, private, mode, reason, &stats, &ret_folios); if (rc_gather < 0) @@ -2137,6 +2164,9 @@ int migrate_pages(struct list_head *from, new_folio_t get_new_folio, if (!list_empty(from)) goto again; out: + if (reason == MR_LONGTERM_PIN) + static_branch_dec(&folio_put_wakeup_key); + /* * Put the permanent failure folio back to migration list, they * will be put back to the right list by the caller. -- 2.53.0