linux-kernel - [PATCH v7 2/4] mm/vmalloc: fix show vmap_area information race with vmap

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Message-Id: <1380070964-12975-2-git-send-email-liwanp@linux.vnet.ibm.com>
Date:	Wed, 25 Sep 2013 09:02:42 +0800
From:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Subject: [PATCH v7 2/4] mm/vmalloc: fix show vmap_area information race with vmap_area tear down

Changelog:
 *v4 -> v5: return directly for !VM_VM_AREA case and remove (VM_LAZY_FREE | VM_LAZY_FREEING) check
 *v5 -> v6: add KOSAKI's ack

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
commit: d4033afd(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@....com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Signed-off-by: Wanpeng Li <liwanp@...ux.vnet.ibm.com>
---
 mm/vmalloc.c |   13 +++++--------
 1 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index f75c2aa..7647e69 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;
 
-- 
1.7.5.4

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