Re: [patch 3/4] timerfd: Implement timerfd_ioctl method to restore timerfd_ctx::ticks

The read() of timerfd files allows to fetch the number of timer ticks
while there is no way to set it back from userspace.

To restore the timer's state as it was at checkpoint moment we need
a path to bring @ticks back. Initially I thought about writing ticks
back via write() interface but it seems such API is somehow obscure.

Instead implement timerfd_ioctl() method with TFD_IOC_SET_TICKS
command which allows to adjust @ticks into non-zero value waking
up the waiters.

I wrapped code with CONFIG_CHECKPOINT_RESTORE which can be
dropped off if there users except c/r camp appear.

v2 (by akpm@):
 -Use define timerfd_ioctl NULL for non c/r config

CC: Thomas Gleixner <redacted>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Andrey Vagin <redacted>
CC: Pavel Emelyanov <redacted>
CC: Vladimir Davydov <redacted>
Signed-off-by: Cyrill Gorcunov <redacted>
---

A nit fixed for for non c/r config in timerfd_ioctl declaration

 fs/timerfd.c            |   37 +++++++++++++++++++++++++++++++++++++
 include/linux/timerfd.h |    5 +++++
 2 files changed, 42 insertions(+)

Index: linux-2.6.git/fs/timerfd.c
===================================================================

--- linux-2.6.git.orig/fs/timerfd.c
+++ linux-2.6.git/fs/timerfd.c

@@ -315,12 +315,49 @@ static int timerfd_show(struct seq_file
 #define timerfd_show NULL
 #endif
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+static long timerfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct timerfd_ctx *ctx = file->private_data;
+	int ret = 0;
+
+	switch (cmd) {
+	case TFD_IOC_SET_TICKS: {
+		u64 ticks;
+
+		if (get_user(ticks, (u64 __user *)arg))