Re: + smaps-add-clear_refs-file-to-clear-reference.patch added to -mm tree

[David Rientjes <rientjes@google.com>]

Do not clear references when the task_struct's mm is NULL by using 
/proc/pid/clear_refs.

Also, use mmap_sem since the mm_struct's VMA's are being iterated in 
fs/proc/task_mmu.c.

Reported by Oleg Nesterov <oleg@tv-sign.ru>.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 fs/proc/base.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -719,6 +719,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 				size_t count, loff_t *ppos)
 {
 	struct task_struct *task;
+	struct mm_struct *mm;
 	char buffer[PROC_NUMBUF], *end;
 
 	memset(buffer, 0, sizeof(buffer));
@@ -733,7 +734,13 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 	task = get_proc_task(file->f_path.dentry->d_inode);
 	if (!task)
 		return -ESRCH;
-	clear_refs_smap(task->mm->mmap);
+	mm = get_task_mm(task);
+	if (mm) {
+		down_read(&mm->mmap_sem);
+		clear_refs_smap(mm->mmap);
+		up_read(&mm->mmap_sem);
+		mmput(mm);
+	}
 	put_task_struct(task);
 	if (end - buffer == 0)
 		return -EIO;
-
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