[PATCH] fs/stat: Reduce memory requirements for stat_open

[Stefan Bader]

When reading /proc/stat the stat_open function currently sizes its
internal buffer at:

  1024 + 128 * num_possible_cpus() + 2 * num_irqs

This is to maximise the chances of the results as returned to userspace
be a single internally consistent result.  With CONFIG_NR_CPUS sized
for larger configs this buffer balloons rapidly, at 256 cpus we end
up at least 33kB which translates into an order-4 allocation (64kB).
This triggered random errors in top when reading /proc/stat due to
memory allocation failures.

In reality the buffer is only consumed in proportion to the
num_online_cpus(), so in the common case it makes much more sense to
allocate the buffer size based on that.  Secondly, regardless of size we
should not be requiring allocations greater than PAGE_ALLOC_COSTLY_ORDER
as allocations above this order are significantly more likely to fail.

As the code already bounds the buffer size based on the maximum kmem_alloc
allocation size, we are already relying on the seq_file buffering when
this is exceeded.  This will also protect us from overflowing should cpus
come online mid read.

We do not attempt to fix potential inconsistancies that this existing
use of this buffering introduces.

BugLink: http://bugs.launchpad.net/bugs/1319244
Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>

---
 fs/proc/stat.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 9d231e9..9498cf7 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -184,7 +184,7 @@ static int show_stat(struct seq_file *p, void *v)
 
 static int stat_open(struct inode *inode, struct file *file)
 {
-	size_t size = 1024 + 128 * num_possible_cpus();
+	size_t size = 1024 + 128 * num_online_cpus();
 	char *buf;
 	struct seq_file *m;
 	int res;
@@ -193,8 +193,8 @@ static int stat_open(struct inode *inode, struct file *file)
 	size += 2 * nr_irqs;
 
 	/* don't ask for more than the kmalloc() max size */
-	if (size > KMALLOC_MAX_SIZE)
-		size = KMALLOC_MAX_SIZE;
+	if (size > (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
+		size = PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER;
 	buf = kmalloc(size, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
-- 
1.7.9.5

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