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Commit 9e781440 authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds
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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.

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: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: default avatarDavid Rientjes <rientjes@google.com>
Acked-by: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3b641bf4

fs/proc/internal.h

+4 −0
Original line number Diff line number Diff line
@@ -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);

fs/proc/task_mmu.c

+47 −3
Original line number Diff line number Diff line
@@ -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);