epoll: drop 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>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Michael Kerrisk <mtk.manpages@googlemail.com>
Cc: Bron Gondwana <brong@fastmail.fm>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 4 insertions, 19 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ba2f9ec7119..011b9b8c90c 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -234,8 +234,6 @@ struct ep_pqueue {
 /*
  * 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 @@ static int zero;
 
 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 @@ static void ep_free(struct eventpoll *ep)
 
         mutex_unlock(&epmutex);
         mutex_destroy(&ep->mtx);
-        atomic_dec(&ep->user->epoll_devs);
         free_uid(ep->user);
         kfree(ep);
 }
@@ -581,10 +570,6 @@ static int ep_alloc(struct eventpoll **pep)
         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 @@ SYSCALL_DEFINE1(epoll_create1, int, flags)
                               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 @@ static int __init eventpoll_init(void)
         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 */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 02e16d20730..5a7c7638873 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -630,7 +630,6 @@ struct user_struct {
         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