From: Hugh Dickins <hughd@xxxxxxxxxx>
When correcting commit 04fa5d6a (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@xxxxxxx: changelog only]
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
mm/huge_memory.c | 28 ++++++----------
mm/migrate.c | 95 ++++++++++++++++++++++++------------------------------
2 files changed, 52 insertions(+), 71 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6001ee6..648c102 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1298,7 +1298,6 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
int target_nid;
int current_nid = -1;
bool migrated;
- bool page_locked = false;
spin_lock(&mm->page_table_lock);
if (unlikely(!pmd_same(pmd, *pmdp)))
@@ -1320,7 +1319,6 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
/* Acquire the page lock to serialise THP migrations */
spin_unlock(&mm->page_table_lock);
lock_page(page);
- page_locked = true;
/* Confirm the PTE did not while locked */
spin_lock(&mm->page_table_lock);
@@ -1333,34 +1331,26 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
/* Migrate the THP to the requested node */
migrated = migrate_misplaced_transhuge_page(mm, vma,
- pmdp, pmd, addr,
- page, target_nid);
- if (migrated)
- current_nid = target_nid;
- else {
- spin_lock(&mm->page_table_lock);
- if (unlikely(!pmd_same(pmd, *pmdp))) {
- unlock_page(page);
- goto out_unlock;
- }
- goto clear_pmdnuma;
- }
+ pmdp, pmd, addr, page, target_nid);
+ if (!migrated)
+ goto check_same;
- task_numa_fault(current_nid, HPAGE_PMD_NR, migrated);
+ task_numa_fault(target_nid, HPAGE_PMD_NR, true);
return 0;
+check_same:
+ spin_lock(&mm->page_table_lock);
+ if (unlikely(!pmd_same(pmd, *pmdp)))
+ goto out_unlock;
clear_pmdnuma:
pmd = pmd_mknonnuma(pmd);
set_pmd_at(mm, haddr, pmdp, pmd);
VM_BUG_ON(pmd_numa(*pmdp));
update_mmu_cache_pmd(vma, addr, pmdp);
- if (page_locked)
- unlock_page(page);
-
out_unlock:
spin_unlock(&mm->page_table_lock);
if (current_nid != -1)
- task_numa_fault(current_nid, HPAGE_PMD_NR, migrated);
+ task_numa_fault(current_nid, HPAGE_PMD_NR, false);
return 0;
}
diff --git a/mm/migrate.c b/mm/migrate.c
index 73e432d..8ef1cbf 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1555,41 +1555,40 @@ 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;
}
/*
@@ -1600,7 +1599,7 @@ 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);
@@ -1608,20 +1607,16 @@ 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)
@@ -1638,12 +1633,19 @@ 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,
@@ -1674,29 +1676,15 @@ 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 */
@@ -1728,6 +1716,7 @@ 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;
}
@@ -1772,9 +1761,11 @@ 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 */
--
1.7.9.2
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