[PATCH] proc: don't lock task_structs indefinitely

Every inode in /proc holds a reference to a struct task_struct.  If a
directory or file is opened and remains open after the the task exits this
pinning continues.  With 8K stacks on a 32bit machine the amount pinned per
file descriptor is about 10K.

Normally I would figure a reasonable per user process limit is about 100
processes.  With 80 processes, with a 1000 file descriptors each I can trigger
the 00M killer on a 32bit kernel, because I have pinned about 800MB of useless
data.

This patch replaces the struct task_struct pointer with a pointer to a struct
task_ref which has a struct task_struct pointer.  The so the pinning of dead
tasks does not happen.

The code now has to contend with the fact that the task may now exit at any
time.  Which is a little but not muh more complicated.

With this change it takes about 1000 processes each opening up 1000 file
descriptors before I can trigger the OOM killer.  Much better.

[mlp@google.com: task_mmu small fixes]
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Paul Jackson <pj@sgi.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Albert Cahalan <acahalan@gmail.com>
Signed-off-by: Prasanna Meda <mlp@google.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 73e0f23..6b9740b 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1821,7 +1821,7 @@ static inline void check_huge_range(struct vm_area_struct *vma,
 
 int show_numa_map(struct seq_file *m, void *v)
 {
-	struct task_struct *task = m->private;
+	struct proc_maps_private *priv = m->private;
 	struct vm_area_struct *vma = v;
 	struct numa_maps *md;
 	struct file *file = vma->vm_file;
@@ -1837,7 +1837,7 @@ int show_numa_map(struct seq_file *m, void *v)
 		return 0;
 
 	mpol_to_str(buffer, sizeof(buffer),
-			get_vma_policy(task, vma, vma->vm_start));
+			    get_vma_policy(priv->task, vma, vma->vm_start));
 
 	seq_printf(m, "%08lx %s", vma->vm_start, buffer);
 
@@ -1891,7 +1891,7 @@ out:
 	kfree(md);
 
 	if (m->count < m->size)
-		m->version = (vma != get_gate_vma(task)) ? vma->vm_start : 0;
+		m->version = (vma != priv->tail_vma) ? vma->vm_start : 0;
 	return 0;
 }