[Added CC to linux-api]
On Thu 08-02-18 18:07:37, Kirill Tkhai wrote:
> Watch descriptor is id of the watch created by inotify_add_watch().
> It is allocated in inotify_add_to_idr(), and takes the numbers
> starting from 1. Every new inotify watch obtains next available
> number (usually, old + 1), as served by idr_alloc_cyclic().
>
> CRIU (Checkpoint/Restore In Userspace) project supports inotify
> files, and restores watched descriptors with the same numbers,
> they had before dump. Since there was no kernel support, we
> had to use cycle to add a watch with specific descriptor id:
>
> while (1) {
> int wd;
>
> wd = inotify_add_watch(inotify_fd, path, mask);
> if (wd < 0) {
> break;
> } else if (wd == desired_wd_id) {
> ret = 0;
> break;
> }
>
> inotify_rm_watch(inotify_fd, wd);
> }
>
> (You may find the actual code at the below link:
> https://github.com/checkpoint-restore/criu/blob/v3.7/criu/fsnotify.c#L577)
>
> The cycle is suboptiomal and very expensive, but since there is no better
> kernel support, it was the only way to restore that. Happily, we had met
> mostly descriptors with small id, and this approach had worked somehow.
>
> But recent time containers with inotify with big watch descriptors
> begun to come, and this way stopped to work at all. When descriptor id
> is something about 0x34d71d6, the restoring process spins in busy loop
> for a long time, and the restore hungs and delay of migration from node
> to node could easily be watched.
>
> This patch aims to solve this problem. It introduces new ioctl
> INOTIFY_IOC_SETNEXTWD, which allows to request the number of next created
> watch descriptor from userspace. It simply calls idr_set_cursor() primitive
> to populate idr::idr_next, so that next idr_alloc_cyclic() allocation
> will return this id, if it is not occupied. This is the way which is
> used to restore some other resources from userspace. For example,
> /proc/sys/kernel/ns_last_pid works the same for task pids.
>
> The new code is under CONFIG_CHECKPOINT_RESTORE #define, so small system
> may exclude it. The only change in generic inotify part is idr_alloc_cyclic()
> end argument. We had 0 there, and idr subsystem replaced it with INT_MAX
> in idr_get_free(). So, the max possible id was INT_MAX (see idr_get_free()
> again).
>
> Since I need INOTIFY_IDR_END to check ioctl's third argument, it's better
> it's defined as positive number. But when not-zero value is passed
> to idr_get_free(), this function decrements it. Also, idr_alloc_cyclic()
> defined @end as int argument. So, it's impossible to pass positive @end
> argument to idr_alloc_cyclic() to get INT_MAX id. And after this patch
> inotify watch descriptors ids will take numbers [1, INT_MAX-1], INT_MAX
> will be unavailable.
>
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
I'm not thrilled by this but OTOH I also don't know about a better option.
The patch as such looks OK to me so unless someone objects I'll take it to
my tree in a few days.
Honza
> ---
> fs/notify/inotify/inotify_user.c | 19 ++++++++++++++++++-
> include/uapi/linux/inotify.h | 8 ++++++++
> 2 files changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
> index 5c29bf16814f..3c824e252c84 100644
> --- a/fs/notify/inotify/inotify_user.c
> +++ b/fs/notify/inotify/inotify_user.c
> @@ -44,6 +44,9 @@
>
> #include <asm/ioctls.h>
>
> +#define INOTIFY_IDR_START 1
> +#define INOTIFY_IDR_END S32_MAX
> +
> /* configurable via /proc/sys/fs/inotify/ */
> static int inotify_max_queued_events __read_mostly;
>
> @@ -285,6 +288,7 @@ static int inotify_release(struct inode *ignored, struct file *file)
> static long inotify_ioctl(struct file *file, unsigned int cmd,
> unsigned long arg)
> {
> + struct inotify_group_private_data *data __maybe_unused;
> struct fsnotify_group *group;
> struct fsnotify_event *fsn_event;
> void __user *p;
> @@ -293,6 +297,7 @@ static long inotify_ioctl(struct file *file, unsigned int cmd,
>
> group = file->private_data;
> p = (void __user *) arg;
> + data = &group->inotify_data;
>
> pr_debug("%s: group=%p cmd=%u\n", __func__, group, cmd);
>
> @@ -307,6 +312,17 @@ static long inotify_ioctl(struct file *file, unsigned int cmd,
> spin_unlock(&group->notification_lock);
> ret = put_user(send_len, (int __user *) p);
> break;
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> + case INOTIFY_IOC_SETNEXTWD:
> + ret = -EINVAL;
> + if (arg >= INOTIFY_IDR_START && arg < INOTIFY_IDR_END) {
> + spin_lock(&data->idr_lock);
> + idr_set_cursor(&data->idr, (unsigned int)arg);
> + spin_unlock(&data->idr_lock);
> + ret = 0;
> + }
> + break;
> +#endif /* CONFIG_CHECKPOINT_RESTORE */
> }
>
> return ret;
> @@ -349,7 +365,8 @@ static int inotify_add_to_idr(struct idr *idr, spinlock_t *idr_lock,
> idr_preload(GFP_KERNEL);
> spin_lock(idr_lock);
>
> - ret = idr_alloc_cyclic(idr, i_mark, 1, 0, GFP_NOWAIT);
> + ret = idr_alloc_cyclic(idr, i_mark, INOTIFY_IDR_START,
> + INOTIFY_IDR_END, GFP_NOWAIT);
> if (ret >= 0) {
> /* we added the mark to the idr, take a reference */
> i_mark->wd = ret;
> diff --git a/include/uapi/linux/inotify.h b/include/uapi/linux/inotify.h
> index 5474461683db..245489342c14 100644
> --- a/include/uapi/linux/inotify.h
> +++ b/include/uapi/linux/inotify.h
> @@ -71,5 +71,13 @@ struct inotify_event {
> #define IN_CLOEXEC O_CLOEXEC
> #define IN_NONBLOCK O_NONBLOCK
>
> +/*
> + * ioctl numbers: inotify uses 'I' prefix for all ioctls,
> + * except historical FIONREAD, which based on 'T'.
> + *
> + * INOTIFY_IOC_SETNEXTWD: set desired number of next created
> + * watch descriptor.
> + */
> +#define INOTIFY_IOC_SETNEXTWD _IOW('I', 0, __s32)
>
> #endif /* _UAPI_LINUX_INOTIFY_H */
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
