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hold task->mempolicy while numa_maps scans.

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/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.

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.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
KAMEZAWA Hiroyuki 2012-10-19 17:00:55 +09:00 committed by Linus Torvalds
parent 3b641bf453
commit 9e7814404b
2 changed files with 51 additions and 3 deletions
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4
fs/proc/internal.h
View file

@ -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);

50
fs/proc/task_mmu.c
View file

@ -90,10 +90,55 @@ static void pad_len_spaces(struct seq_file *m, int len)
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 +177,7 @@ 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 +204,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
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 +1224,9 @@ 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);