From 42836f5f8baa33085f547098b74aa98991ee9216 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Mon, 7 Oct 2013 11:28:43 +0100 Subject: [PATCH] mm: Wait for THP migrations to complete during NUMA hinting faults The locking for migrating THP is unusual. While normal page migration prevents parallel accesses using a migration PTE, THP migration relies on a combination of the page_table_lock, the page lock and the existance of the NUMA hinting PTE to guarantee safety but there is a bug in the scheme. If a THP page is currently being migrated and another thread traps a fault on the same page it checks if the page is misplaced. If it is not, then pmd_numa is cleared. The problem is that it checks if the page is misplaced without holding the page lock meaning that the racing thread can be migrating the THP when the second thread clears the NUMA bit and faults a stale page. This patch checks if the page is potentially being migrated and stalls using the lock_page if it is potentially being migrated before checking if the page is misplaced or not. Signed-off-by: Mel Gorman Reviewed-by: Rik van Riel Cc: Andrea Arcangeli Cc: Johannes Weiner Cc: Srikar Dronamraju Cc: Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/1381141781-10992-6-git-send-email-mgorman@suse.de Signed-off-by: Ingo Molnar --- mm/huge_memory.c | 23 ++++++++++++++++------- 1 file changed, 16 insertions(+), 7 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 33ee637648ba..e10d780c4781 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1295,13 +1295,14 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, if (current_nid == numa_node_id()) count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL); - target_nid = mpol_misplaced(page, vma, haddr); - if (target_nid == -1) { - put_page(page); - goto clear_pmdnuma; - } + /* + * Acquire the page lock to serialise THP migrations but avoid dropping + * page_table_lock if at all possible + */ + if (trylock_page(page)) + goto got_lock; - /* Acquire the page lock to serialise THP migrations */ + /* Serialise against migrationa and check placement check placement */ spin_unlock(&mm->page_table_lock); lock_page(page); @@ -1312,9 +1313,17 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, put_page(page); goto out_unlock; } - spin_unlock(&mm->page_table_lock); + +got_lock: + target_nid = mpol_misplaced(page, vma, haddr); + if (target_nid == -1) { + unlock_page(page); + put_page(page); + goto clear_pmdnuma; + } /* Migrate the THP to the requested node */ + spin_unlock(&mm->page_table_lock); migrated = migrate_misplaced_transhuge_page(mm, vma, pmdp, pmd, addr, page, target_nid); if (!migrated)