+ mm-migrate-check-mapcount-for-thp-instead-of-refcount.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: mm: migrate: check mapcount for THP instead of refcount
has been added to the -mm tree.  Its filename is
     mm-migrate-check-mapcount-for-thp-instead-of-refcount.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-migrate-check-mapcount-for-thp-instead-of-refcount.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-migrate-check-mapcount-for-thp-instead-of-refcount.patch

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------------------------------------------------------
From: Yang Shi <shy828301@gmail.com>
Subject: mm: migrate: check mapcount for THP instead of refcount

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.

Link: https://lkml.kernel.org/r/20210518200801.7413-7-shy828301@gmail.com
Signed-off-by: Yang Shi <shy828301@gmail.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/migrate.c |   16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

--- a/mm/migrate.c~mm-migrate-check-mapcount-for-thp-instead-of-refcount
+++ a/mm/migrate.c
@@ -2072,6 +2072,10 @@ static int numamigrate_isolate_page(pg_d
 
 	VM_BUG_ON_PAGE(compound_order(page) && !PageTransHuge(page), page);
 
+	/* Do not migrate THP mapped by multiple processes */
+	if (PageTransHuge(page) && total_mapcount(page) > 1)
+		return 0;
+
 	/* Avoid migrating to a node that is nearly full */
 	if (!migrate_balanced_pgdat(pgdat, compound_nr(page)))
 		return 0;
@@ -2079,18 +2083,6 @@ static int numamigrate_isolate_page(pg_d
 	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));
_

Patches currently in -mm which might be from shy828301@gmail.com are

mm-thp-check-total_mapcount-instead-of-page_mapcount.patch
mm-memory-add-orig_pmd-to-struct-vm_fault.patch
mm-memory-make-numa_migrate_prep-non-static.patch
mm-thp-refactor-numa-fault-handling.patch
mm-migrate-account-thp-numa-migration-counters-correctly.patch
mm-migrate-dont-split-thp-for-misplaced-numa-page.patch
mm-migrate-check-mapcount-for-thp-instead-of-refcount.patch
mm-thp-skip-make-pmd-prot_none-if-thp-migration-is-not-supported.patch