[PATCH v2] inotify: Increase default inotify.max_user_watches limit to 1048576

[Waiman Long]

The default value of inotify.max_user_watches sysctl parameter was set
to 8192 since the introduction of the inotify feature in 2005 by
commit 0eeca28300df ("[PATCH] inotify"). Today this value is just too
small for many modern usage. As a result, users have to explicitly set
it to a larger value to make it work.

After some searching around the web, these are the
inotify.max_user_watches values used by some projects:
 - vscode:  524288
 - dropbox support: 100000
 - users on stackexchange: 12228
 - lsyncd user: 2000000
 - code42 support: 1048576
 - monodevelop: 16384
 - tectonic: 524288
 - openshift origin: 65536

Each watch point adds an inotify_inode_mark structure to an inode to
be watched. It also pins the watched inode as well as an inotify fdinfo
procfs file.

Modeled after the epoll.max_user_watches behavior to adjust the default
value according to the amount of addressable memory available, make
inotify.max_user_watches behave in a similar way to make it use no more
than 1% of addressable memory within the range [8192, 1048576].

For 64-bit archs, inotify_inode_mark plus 2 inode have a size close
to 2 kbytes. That means a system with 196GB or more memory should have
the maximum value of 1048576 for inotify.max_user_watches. This default
should be big enough for most use cases.

With my x86-64 config, the size of xfs_inode, proc_inode and
inotify_inode_mark is 1680 bytes. The estimated INOTIFY_WATCH_COST is
1760 bytes.

[v2: increase inotify watch cost as suggested by Amir and Honza]

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 fs/notify/inotify/inotify_user.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 186722ba3894..37d9f09c226f 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -37,6 +37,16 @@
 
 #include <asm/ioctls.h>
 
+/*
+ * An inotify watch requires allocating an inotify_inode_mark structure as
+ * well as pinning the watched inode and adding inotify fdinfo procfs file.
+ * The increase in size of a filesystem inode versus a VFS inode varies
+ * depending on the filesystem. An extra 512 bytes is added as rough
+ * estimate of the additional filesystem inode cost.
+ */
+#define INOTIFY_WATCH_COST	(sizeof(struct inotify_inode_mark) + \
+				 2 * sizeof(struct inode) + 512)
+
 /* configurable via /proc/sys/fs/inotify/ */
 static int inotify_max_queued_events __read_mostly;
 
@@ -801,6 +811,18 @@ SYSCALL_DEFINE2(inotify_rm_watch, int, fd, __s32, wd)
  */
 static int __init inotify_user_setup(void)
 {
+	unsigned int watches_max;
+	struct sysinfo si;
+
+	si_meminfo(&si);
+	/*
+	 * Allow up to 1% of addressible memory to be allocated for inotify
+	 * watches (per user) limited to the range [8192, 1048576].
+	 */
+	watches_max = (((si.totalram - si.totalhigh) / 100) << PAGE_SHIFT) /
+			INOTIFY_WATCH_COST;
+	watches_max = min(1048576U, max(watches_max, 8192U));
+
 	BUILD_BUG_ON(IN_ACCESS != FS_ACCESS);
 	BUILD_BUG_ON(IN_MODIFY != FS_MODIFY);
 	BUILD_BUG_ON(IN_ATTRIB != FS_ATTRIB);
@@ -827,7 +849,7 @@ static int __init inotify_user_setup(void)
 
 	inotify_max_queued_events = 16384;
 	init_user_ns.ucount_max[UCOUNT_INOTIFY_INSTANCES] = 128;
-	init_user_ns.ucount_max[UCOUNT_INOTIFY_WATCHES] = 8192;
+	init_user_ns.ucount_max[UCOUNT_INOTIFY_WATCHES] = watches_max;
 
 	return 0;
 }
-- 
2.18.1