lkml.org 
[lkml]   [2012]   [Oct]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside spinlock when reading numa_maps
(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@jp.fujitsu.com>
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@jp.fujitsu.com>

V1->V2
- access task->mempolicy only once and remember it. Becase kernel/exit.c
can overwrite it.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
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;
#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..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, ' ');
}
+#ifdef CONFIG_NUMA
+/*
+ * 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);
+}
+#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 +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;
up_read(&mm->mmap_sem);
--
1.7.10.2




\
 
 \ /
  Last update: 2012-10-18 07:21    [W:0.084 / U:1.996 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site