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Date: Mon, 22 Oct 2012 11:47:31 +0900
From: Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: David Rientjes <rientjes@...gle.com>
CC: Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Dave Jones <davej@...hat.com>,
KOSAKI Motohiro <kosaki.motohiro@...il.com>,
bhutchings@...arflare.com,
Konstantin Khlebnikov <khlebnikov@...nvz.org>,
Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [patch for-3.7 v3] mm, mempolicy: hold task->mempolicy refcount
while reading numa_maps.
(2012/10/19 18:28), David Rientjes wrote:
> Looks good, but the patch is whitespace damaged so it doesn't apply. When
> that's fixed:
>
> Acked-by: David Rientjes <rientjes@...gle.com>
Sorry, I hope this one is not broken...
==
From c5849c9034abeec3f26bf30dadccd393b0c5c25e Mon Sep 17 00:00:00 2001
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Date: Fri, 19 Oct 2012 17:00:55 +0900
Subject: [PATCH] hold task->mempolicy while numa_maps scans.
/proc/<pid>/numa_maps scans vma and show mempolicy under
mmap_sem. It sometimes accesses task->mempolicy which can
be freed without mmap_sem and numa_maps can show some
garbage while scanning.
This patch tries to take reference count of task->mempolicy at reading
numa_maps before calling get_vma_policy(). By this, task->mempolicy
will not be freed until numa_maps reaches its end.
Acked-by: David Rientjes <rientjes@...gle.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
V2->v3
- updated comments to be more verbose.
- removed task_lock() in numa_maps code.
V1->V2
- access task->mempolicy only once and remember it. Becase kernel/exit.c
can overwrite it.
---
fs/proc/internal.h | 4 ++++
fs/proc/task_mmu.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
2 files changed, 50 insertions(+), 3 deletions(-)
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index cceaab0..43973b0 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -12,6 +12,7 @@
#include <linux/sched.h>
#include <linux/proc_fs.h>
struct ctl_table_header;
+struct mempolicy;
extern struct proc_dir_entry proc_root;
#ifdef CONFIG_PROC_SYSCTL
@@ -74,6 +75,9 @@ struct proc_maps_private {
#ifdef CONFIG_MMU
struct vm_area_struct *tail_vma;
#endif
+#ifdef CONFIG_NUMA
+ struct mempolicy *task_mempolicy;
+#endif
};
void proc_init_inodecache(void);
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 14df880..2371fea 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -89,11 +89,55 @@ static void pad_len_spaces(struct seq_file *m, int len)
len = 1;
seq_printf(m, "%*c", len, ' ');
}
+#ifdef CONFIG_NUMA
+/*
+ * These functions are for numa_maps but called in generic **maps seq_file
+ * ->start(), ->stop() ops.
+ *
+ * numa_maps scans all vmas under mmap_sem and checks their mempolicy.
+ * Each mempolicy object is controlled by reference counting. The problem here
+ * is how to avoid accessing dead mempolicy object.
+ *
+ * Because we're holding mmap_sem while reading seq_file, it's safe to access
+ * each vma's mempolicy, no vma objects will never drop refs to mempolicy.
+ *
+ * A task's mempolicy (task->mempolicy) has different behavior. task->mempolicy
+ * is set and replaced under mmap_sem but unrefed and cleared under task_lock().
+ * So, without task_lock(), we cannot trust get_vma_policy() because we cannot
+ * gurantee the task never exits under us. But taking task_lock() around
+ * get_vma_plicy() causes lock order problem.
+ *
+ * To access task->mempolicy without lock, we hold a reference count of an
+ * object pointed by task->mempolicy and remember it. This will guarantee
+ * that task->mempolicy points to an alive object or NULL in numa_maps accesses.
+ */
+static void hold_task_mempolicy(struct proc_maps_private *priv)
+{
+ struct task_struct *task = priv->task;
+
+ task_lock(task);
+ priv->task_mempolicy = task->mempolicy;
+ mpol_get(priv->task_mempolicy);
+ task_unlock(task);
+}
+static void release_task_mempolicy(struct proc_maps_private *priv)
+{
+ mpol_put(priv->task_mempolicy);
+}
+#else
+static void hold_task_mempolicy(struct proc_maps_private *priv)
+{
+}
+static void release_task_mempolicy(struct proc_maps_private *priv)
+{
+}
+#endif
static void vma_stop(struct proc_maps_private *priv, struct vm_area_struct *vma)
{
if (vma && vma != priv->tail_vma) {
struct mm_struct *mm = vma->vm_mm;
+ release_task_mempolicy(priv);
up_read(&mm->mmap_sem);
mmput(mm);
}
@@ -132,7 +176,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
tail_vma = get_gate_vma(priv->task->mm);
priv->tail_vma = tail_vma;
-
+ hold_task_mempolicy(priv);
/* Start with last addr hint */
vma = find_vma(mm, last_addr);
if (last_addr && vma) {
@@ -159,6 +203,7 @@ out:
if (vma)
return vma;
+ release_task_mempolicy(priv);
/* End of vmas has been reached */
m->version = (tail_vma != NULL)? 0: -1UL;
up_read(&mm->mmap_sem);
@@ -1178,11 +1223,9 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
walk.private = md;
walk.mm = mm;
- task_lock(task);
pol = get_vma_policy(task, vma, vma->vm_start);
mpol_to_str(buffer, sizeof(buffer), pol, 0);
mpol_cond_put(pol);
- task_unlock(task);
seq_printf(m, "%08lx %s", vma->vm_start, buffer);
--
1.7.10.2
--
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