mm: Wait for THP migrations to complete during NUMA hinting faults (ff9042b1) · Commits · 戴 / test

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 <mgorman@suse.de> Reviewed-by:

Rik van Riel <riel@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Signed-off-by:

Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/r/1381141781-10992-6-git-send-email-mgorman@suse.de Signed-off-by:

Ingo Molnar <mingo@kernel.org>

mm/huge_memory.c

+16 −7

Original line number	Original line	Diff line number	Diff line
	@@ -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())		if (current_nid == numa_node_id())
	count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);		count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);

	target_nid = mpol_misplaced(page, vma, haddr);		/*
	if (target_nid == -1) {		* Acquire the page lock to serialise THP migrations but avoid dropping
	put_page(page);		* page_table_lock if at all possible
	goto clear_pmdnuma;		*/
	}		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);		spin_unlock(&mm->page_table_lock);
	lock_page(page);		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);		put_page(page);
	goto out_unlock;		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 */		/* Migrate the THP to the requested node */
			spin_unlock(&mm->page_table_lock);
	migrated = migrate_misplaced_transhuge_page(mm, vma,		migrated = migrate_misplaced_transhuge_page(mm, vma,
	pmdp, pmd, addr, page, target_nid);		pmdp, pmd, addr, page, target_nid);
	if (!migrated)		if (!migrated)

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