On 09.02.2018 23:56, Andrew Morton wrote:
> On Fri, 9 Feb 2018 18:04:54 +0300 Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> 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.
>>
>
> Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>
> With a little cleanup:
>
> --- a/fs/notify/inotify/inotify_user.c~inotify-extend-ioctl-to-allow-to-request-id-of-new-watch-descriptor-fix
> +++ a/fs/notify/inotify/inotify_user.c
> @@ -285,7 +285,6 @@ static int inotify_release(struct inode
> 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;
> @@ -294,7 +293,6 @@ static long inotify_ioctl(struct file *f
>
> group = file->private_data;
> p = (void __user *) arg;
> - data = &group->inotify_data;
>
> pr_debug("%s: group=%p cmd=%u\n", __func__, group, cmd);
>
> @@ -313,6 +311,9 @@ static long inotify_ioctl(struct file *f
> case INOTIFY_IOC_SETNEXTWD:
> ret = -EINVAL;
> if (arg >= 1 && arg <= INT_MAX) {
> + struct inotify_group_private_data *data;
> +
> + data = &group->inotify_data;
> spin_lock(&data->idr_lock);
> idr_set_cursor(&data->idr, (unsigned int)arg);
> spin_unlock(&data->idr_lock);
I have no objections.
Thanks,
Kirill
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