mm: numa: cleanup flow of transhuge page migration

When correcting commit 04fa5d6a6547 ("mm: migrate: check page_count of
THP before migrating") Hugh Dickins noted that the control flow for
transhuge migration was difficult to follow.  Unconditionally calling
put_page() in numamigrate_isolate_page() made the failure paths of both
migrate_misplaced_transhuge_page() and migrate_misplaced_page() more
complex that they should be.  Further, he was extremely wary that an
unlock_page() should ever happen after a put_page() even if the
put_page() should never be the final put_page.

Hugh implemented the following cleanup to simplify the path by calling
putback_lru_page() inside numamigrate_isolate_page() if it failed to
isolate and always calling unlock_page() within
migrate_misplaced_transhuge_page().

There is no functional change after this patch is applied but the code
is easier to follow and unlock_page() always happens before put_page().

[mgorman@suse.de: changelog only]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Simon Jeons <simon.jeons@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 43 insertions, 52 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 77f4e70df24d..f560071e89c5 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1557,41 +1557,40 @@ bool numamigrate_update_ratelimit(pg_data_t *pgdat, unsigned long nr_pages)
 
 int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
 {
-        int ret = 0;
+        int page_lru;
 
         VM_BUG_ON(compound_order(page) && !PageTransHuge(page));
 
         /* Avoid migrating to a node that is nearly full */
-        if (migrate_balanced_pgdat(pgdat, 1UL << compound_order(page))) {
-                int page_lru;
+        if (!migrate_balanced_pgdat(pgdat, 1UL << compound_order(page)))
+                return 0;
 
-                if (isolate_lru_page(page)) {
-                        put_page(page);
-                        return 0;
-                }
+        if (isolate_lru_page(page))
+                return 0;
 
-                /* Page is isolated */
-                ret = 1;
-                page_lru = page_is_file_cache(page);
-                if (!PageTransHuge(page))
-                        inc_zone_page_state(page, NR_ISOLATED_ANON + page_lru);
-                else
-                        mod_zone_page_state(page_zone(page),
-                                        NR_ISOLATED_ANON + page_lru,
-                                        HPAGE_PMD_NR);
+        /*
+         * 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_cache(page);
+        mod_zone_page_state(page_zone(page), NR_ISOLATED_ANON + page_lru,
+                                hpage_nr_pages(page));
+
         /*
-         * Page is either isolated or there is not enough space on the target
-         * node. If isolated, then it has taken a reference count and the
-         * callers reference can be safely dropped without the page
-         * disappearing underneath us during migration. Otherwise the page is
-         * not to be migrated but the callers reference should still be
-         * dropped so it does not leak.
+         * Isolating the page has taken another reference, so the
+         * caller's reference can be safely dropped without the page
+         * disappearing underneath us during migration.
          */
         put_page(page);
-
-        return ret;
+        return 1;
 }
 
 /*
@@ -1602,7 +1601,7 @@ int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
 int migrate_misplaced_page(struct page *page, int node)
 {
         pg_data_t *pgdat = NODE_DATA(node);
-        int isolated = 0;
+        int isolated;
         int nr_remaining;
         LIST_HEAD(migratepages);
 
@@ -1610,20 +1609,16 @@ int migrate_misplaced_page(struct page *page, int node)
          * Don't migrate pages that are mapped in multiple processes.
          * TODO: Handle false sharing detection instead of this hammer
          */
-        if (page_mapcount(page) != 1) {
-                put_page(page);
+        if (page_mapcount(page) != 1)
                 goto out;
-        }
 
         /*
          * Rate-limit the amount of data that is being migrated to a node.
          * Optimal placement is no good if the memory bus is saturated and
          * all the time is being spent migrating!
          */
-        if (numamigrate_update_ratelimit(pgdat, 1)) {
-                put_page(page);
+        if (numamigrate_update_ratelimit(pgdat, 1))
                 goto out;
-        }
 
         isolated = numamigrate_isolate_page(pgdat, page);
         if (!isolated)
@@ -1640,12 +1635,19 @@ int migrate_misplaced_page(struct page *page, int node)
         } else
                 count_vm_numa_event(NUMA_PAGE_MIGRATE);
         BUG_ON(!list_empty(&migratepages));
-out:
         return isolated;
+
+out:
+        put_page(page);
+        return 0;
 }
 #endif /* CONFIG_NUMA_BALANCING */
 
 #if defined(CONFIG_NUMA_BALANCING) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
+/*
+ * Migrates a THP to a given target node. page must be locked and is unlocked
+ * before returning.
+ */
 int migrate_misplaced_transhuge_page(struct mm_struct *mm,
                                 struct vm_area_struct *vma,
                                 pmd_t *pmd, pmd_t entry,
@@ -1676,29 +1678,15 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 
         new_page = alloc_pages_node(node,
                 (GFP_TRANSHUGE | GFP_THISNODE) & ~__GFP_WAIT, HPAGE_PMD_ORDER);
-        if (!new_page) {
-                count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
-                goto out_dropref;
-        }
+        if (!new_page)
+                goto out_fail;
+
         page_xchg_last_nid(new_page, page_last_nid(page));
 
         isolated = numamigrate_isolate_page(pgdat, page);
-
-        /*
-         * Failing to isolate or a GUP pin prevents migration. The expected
-         * page count is 2. 1 for anonymous pages without a mapping and 1
-         * for the callers pin. If the page was isolated, the page will
-         * need to be put back on the LRU.
-         */
-        if (!isolated || page_count(page) != 2) {
-                count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
+        if (!isolated) {
                 put_page(new_page);
-                if (isolated) {
-                        putback_lru_page(page);
-                        isolated = 0;
-                        goto out;
-                }
-                goto out_keep_locked;
+                goto out_fail;
         }
 
         /* Prepare a page as a migration target */
@@ -1730,6 +1718,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
                 putback_lru_page(page);
 
                 count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
+                isolated = 0;
                 goto out;
         }
 
@@ -1774,9 +1763,11 @@ out:
                         -HPAGE_PMD_NR);
         return isolated;
 
+out_fail:
+        count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
 out_dropref:
+        unlock_page(page);
         put_page(page);
-out_keep_locked:
         return 0;
 }
 #endif /* CONFIG_NUMA_BALANCING */