mm/vmalloc: fix show vmap_area information race with vmap_area tear down

There is a race window between vmap_area tear down and show vmap_area
information.

	A                                                B

remove_vm_area
spin_lock(&vmap_area_lock);
va->vm = NULL;
va->flags &= ~VM_VM_AREA;
spin_unlock(&vmap_area_lock);
						spin_lock(&vmap_area_lock);
						if (va->flags & (VM_LAZY_FREE | VM_LAZY_FREEZING))
							return 0;
						if (!(va->flags & VM_VM_AREA)) {
							seq_printf(m, "0x%pK-0x%pK %7ld vm_map_ram\n",
								(void *)va->va_start, (void *)va->va_end,
								va->va_end - va->va_start);
							return 0;
						}
free_unmap_vmap_area(va);
	flush_cache_vunmap
	free_unmap_vmap_area_noflush
		unmap_vmap_area
		free_vmap_area_noflush
			va->flags |= VM_LAZY_FREE

The assumption !VM_VM_AREA represents vm_map_ram allocation is
introduced by d4033afdf828 ("mm, vmalloc: iterate vmap_area_list,
instead of vmlist, in vmallocinfo()").

However, !VM_VM_AREA also represents vmap_area is being tear down in
race window mentioned above.  This patch fix it by don't dump any
information for !VM_VM_AREA case and also remove (VM_LAZY_FREE |
VM_LAZY_FREEING) check since they are not possible for !VM_VM_AREA case.

Suggested-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Cc: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 8 deletions

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 285f0e7d28e7..814ce9122709 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2578,15 +2578,12 @@ static int s_show(struct seq_file *m, void *p)
         struct vmap_area *va = p;
         struct vm_struct *v;
 
-        if (va->flags & (VM_LAZY_FREE | VM_LAZY_FREEING))
-                return 0;
-
-        if (!(va->flags & VM_VM_AREA)) {
-                seq_printf(m, "0x%pK-0x%pK %7ld vm_map_ram\n",
-                        (void *)va->va_start, (void *)va->va_end,
-                                        va->va_end - va->va_start);
+        /*
+         * s_show can encounter race with remove_vm_area, !VM_VM_AREA on
+         * behalf of vmap area is being tear down or vm_map_ram allocation.
+         */
+        if (!(va->flags & VM_VM_AREA))
                 return 0;
-        }
 
         v = va->vm;