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Subject[PATCH 8/8] proc: allocate storage for numa_maps statistics once
Date
In show_numa_map() we collect statistics into a numa_maps structure.
Since the number of NUMA nodes can be very large, this structure is not
a candidate for stack allocation.

Instead of going thru a kmalloc()+kfree() cycle each time show_numa_map()
is invoked, perform the allocation just once when /proc/pid/numa_maps is
opened.

Performing the allocation when numa_maps is opened, and thus before a
reference to the target tasks mm is taken, eliminates a potential
stalemate condition in the oom-killer as originally described by Hugh
Dickins:

... imagine what happens if the system is out of memory, and the mm
we're looking at is selected for killing by the OOM killer: while
we wait in __get_free_page for more memory, no memory is freed
from the selected mm because it cannot reach exit_mmap while we hold
that reference.

Signed-off-by: Stephen Wilson <wilsons@start.ca>
---
fs/proc/task_mmu.c | 36 +++++++++++++++++++++++++++---------
1 files changed, 27 insertions(+), 9 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 9f069d2..1ca3a00 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -868,6 +868,11 @@ struct numa_maps {
unsigned long node[MAX_NUMNODES];
};

+struct numa_maps_private {
+ struct proc_maps_private proc_maps;
+ struct numa_maps md;
+};
+
static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty)
{
int count = page_mapcount(page);
@@ -949,9 +954,10 @@ static int gather_hugetbl_stats(pte_t *pte, unsigned long hmask,
*/
static int show_numa_map(struct seq_file *m, void *v)
{
- struct proc_maps_private *priv = m->private;
+ struct numa_maps_private *numa_priv = m->private;
+ struct proc_maps_private *proc_priv = &numa_priv->proc_maps;
struct vm_area_struct *vma = v;
- struct numa_maps *md;
+ struct numa_maps *md = &numa_priv->md;
struct file *file = vma->vm_file;
struct mm_struct *mm = vma->vm_mm;
struct mm_walk walk = {};
@@ -962,16 +968,15 @@ static int show_numa_map(struct seq_file *m, void *v)
if (!mm)
return 0;

- md = kzalloc(sizeof(struct numa_maps), GFP_KERNEL);
- if (!md)
- return 0;
+ /* Ensure we start with an empty set of numa_maps statistics. */
+ memset(md, 0, sizeof(*md));

walk.hugetlb_entry = gather_hugetbl_stats;
walk.pte_entry = gather_pte_stats;
walk.private = md;
walk.mm = mm;

- pol = get_vma_policy(priv->task, vma, vma->vm_start);
+ pol = get_vma_policy(proc_priv->task, vma, vma->vm_start);
mpol_to_str(buffer, sizeof(buffer), pol, 0);
mpol_cond_put(pol);

@@ -1018,12 +1023,12 @@ static int show_numa_map(struct seq_file *m, void *v)
seq_printf(m, " N%d=%lu", n, md->node[n]);
out:
seq_putc(m, '\n');
- kfree(md);

if (m->count < m->size)
- m->version = (vma != priv->tail_vma) ? vma->vm_start : 0;
+ m->version = (vma != proc_priv->tail_vma) ? vma->vm_start : 0;
return 0;
}
+
static const struct seq_operations proc_pid_numa_maps_op = {
.start = m_start,
.next = m_next,
@@ -1033,7 +1038,20 @@ static const struct seq_operations proc_pid_numa_maps_op = {

static int numa_maps_open(struct inode *inode, struct file *file)
{
- return do_maps_open(inode, file, &proc_pid_numa_maps_op);
+ struct numa_maps_private *priv;
+ int ret = -ENOMEM;
+ priv = kzalloc(sizeof(*priv), GFP_KERNEL);
+ if (priv) {
+ priv->proc_maps.pid = proc_pid(inode);
+ ret = seq_open(file, &proc_pid_numa_maps_op);
+ if (!ret) {
+ struct seq_file *m = file->private_data;
+ m->private = priv;
+ } else {
+ kfree(priv);
+ }
+ }
+ return ret;
}

const struct file_operations proc_numa_maps_operations = {
--
1.7.3.5


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