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Subject[patch] drop epoll max_user_instances and rely only on max_user_watches
Linus suggested to put limits where the money is, and max_user_watches 
already does that w/out the need of max_user_instances. That has the
advantage to mitigate the potential DoS while allowing pretty generous
default behavior.
Allowing top 4% of low memory (per user) to be allocated in epoll
watches, we have:

LOMEM MAX_WATCHES (per user)
512MB ~178000
1GB ~356000
2GB ~712000

A box with 512MB of lomem, will meet some challenge in hitting 180K
watches, socket buffers math teaches us.
No more max_user_instances limits then.



Signed-off-by: Davide Libenzi <davidel@xmailserver.org>


- Davide


---
fs/eventpoll.c | 22 ++++------------------
include/linux/sched.h | 1 -
2 files changed, 4 insertions(+), 19 deletions(-)
Index: linux-2.6.mod/fs/eventpoll.c
===================================================================
--- linux-2.6.mod.orig/fs/eventpoll.c 2009-01-28 20:06:55.000000000 -0800
+++ linux-2.6.mod/fs/eventpoll.c 2009-01-28 20:08:16.000000000 -0800
@@ -234,8 +234,6 @@
/*
* Configuration options available inside /proc/sys/fs/epoll/
*/
-/* Maximum number of epoll devices, per user */
-static int max_user_instances __read_mostly;
/* Maximum number of epoll watched descriptors, per user */
static int max_user_watches __read_mostly;

@@ -261,14 +259,6 @@

ctl_table epoll_table[] = {
{
- .procname = "max_user_instances",
- .data = &max_user_instances,
- .maxlen = sizeof(int),
- .mode = 0644,
- .proc_handler = &proc_dointvec_minmax,
- .extra1 = &zero,
- },
- {
.procname = "max_user_watches",
.data = &max_user_watches,
.maxlen = sizeof(int),
@@ -491,7 +481,6 @@

mutex_unlock(&epmutex);
mutex_destroy(&ep->mtx);
- atomic_dec(&ep->user->epoll_devs);
free_uid(ep->user);
kfree(ep);
}
@@ -581,10 +570,6 @@
struct eventpoll *ep;

user = get_current_user();
- error = -EMFILE;
- if (unlikely(atomic_read(&user->epoll_devs) >=
- max_user_instances))
- goto free_uid;
error = -ENOMEM;
ep = kzalloc(sizeof(*ep), GFP_KERNEL);
if (unlikely(!ep))
@@ -1141,7 +1126,6 @@
flags & O_CLOEXEC);
if (fd < 0)
ep_free(ep);
- atomic_inc(&ep->user->epoll_devs);

error_return:
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %d\n",
@@ -1366,8 +1350,10 @@
struct sysinfo si;

si_meminfo(&si);
- max_user_instances = 128;
- max_user_watches = (((si.totalram - si.totalhigh) / 32) << PAGE_SHIFT) /
+ /*
+ * Allows top 4% of lomem to be allocated for epoll watches (per user).
+ */
+ max_user_watches = (((si.totalram - si.totalhigh) / 25) << PAGE_SHIFT) /
EP_ITEM_COST;

/* Initialize the structure used to perform safe poll wait head wake ups */
Index: linux-2.6.mod/include/linux/sched.h
===================================================================
--- linux-2.6.mod.orig/include/linux/sched.h 2009-01-28 20:07:41.000000000 -0800
+++ linux-2.6.mod/include/linux/sched.h 2009-01-28 20:08:16.000000000 -0800
@@ -630,7 +630,6 @@
atomic_t inotify_devs; /* How many inotify devs does this user have opened? */
#endif
#ifdef CONFIG_EPOLL
- atomic_t epoll_devs; /* The number of epoll descriptors currently open */
atomic_t epoll_watches; /* The number of file descriptors currently watched */
#endif
#ifdef CONFIG_POSIX_MQUEUE

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