Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside spinlockwhen reading numa_maps

From: Kamezawa Hiroyuki
Date: Thu Oct 18 2012 - 00:34:31 EST

(2012/10/18 13:06), Kamezawa Hiroyuki wrote:
(2012/10/18 6:31), David Rientjes wrote:
As a result of commit 32f8516a8c73 ("mm, mempolicy: fix printing stack
contents in numa_maps"), the mutex protecting a shared policy can be
inadvertently taken while holding task_lock(task).

Recently, commit b22d127a39dd ("mempolicy: fix a race in
shared_policy_replace()") switched the spinlock within a shared policy to
a mutex so sp_alloc() could block. Thus, a refcount must be grabbed on
all mempolicies returned by get_vma_policy() so it isn't freed while being
passed to mpol_to_str() when reading /proc/pid/numa_maps.

This patch only takes task_lock() while dereferencing task->mempolicy in
get_vma_policy() if it's non-NULL in the lockess check to increment its
refcount. This ensures it will remain in memory until dropped by
__mpol_put() after mpol_to_str() is called.

Refcounts of shared policies are grabbed by the ->get_policy() function of
the vma, all others will be grabbed directly in get_vma_policy(). Now
that this is done, all callers now unconditionally drop the refcount.

please add original problem description....

from your 1st patch.
When reading /proc/pid/numa_maps, it's possible to return the contents of
the stack where the mempolicy string should be printed if the policy gets
freed from beneath us.

This happens because mpol_to_str() may return an error the
stack-allocated buffer is then printed without ever being stored.

Hmm, I've read the whole thread again...and, I'm sorry if I misunderstand something.

I think Kosaki mentioned the commit 52cd3b0740. It avoids refcounting in get_vma_policy()
because it's called every time alloc_pages_vma() is called, at every page fault.
So, it seems he doesn't agree this fix because of performance concern on big NUMA,

Can't we have another way to fix ? like this ? too ugly ?
Again, I'm sorry if I misunderstand the points.

Sorry this patch itself may be buggy. please don't test..
I missed that kernel/exit.c sets task->mempolicy to be NULL.
fixed one here.

From 5581c71e68a7f50e52fd67cca00148911023f9f5 Mon Sep 17 00:00:00 2001
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Date: Thu, 18 Oct 2012 13:50:29 +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.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

- access task->mempolicy only once and remember it. Becase kernel/exit.c
can overwrite it.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
fs/proc/internal.h | 4 ++++
fs/proc/task_mmu.c | 33 ++++++++++++++++++++++++++++++++-
2 files changed, 36 insertions(+), 1 deletion(-)

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;
@@ -74,6 +75,9 @@ struct proc_maps_private {
struct vm_area_struct *tail_vma;
+ struct mempolicy *task_mempolicy;
void proc_init_inodecache(void);
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 14df880..624927d 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -89,11 +89,41 @@ static void pad_len_spaces(struct seq_file *m, int len)
len = 1;
seq_printf(m, "%*c", len, ' ');
+ * numa_maps scans all vmas under mmap_sem and checks their mempolicy.
+ * But task->mempolicy is not guarded by mmap_sem, it can be cleared/freed
+ * under task_lock() (see kernel/exit.c) replacement of it is guarded by
+ * mmap_sem. So, take referenceount under task_lock() before we start
+ * scanning and drop it when numa_maps reaches the end.
+ */
+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);
+static void hold_task_mempolicy(struct proc_maps_private *priv)
+static void release_task_mempolicy(struct proc_maps_private *priv)
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);
@@ -132,7 +162,7 @@ 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 +189,7 @@ out:
if (vma)
return vma;
+ release_task_mempolicy(priv);
/* End of vmas has been reached */
m->version = (tail_vma != NULL)? 0: -1UL;

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