Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

[Miklos Szeredi]

On Tue, Mar 3, 2020 at 10:13 AM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>
> > I'm doing a patch.   Let's see how it fares in the face of all these
> > preconceptions.
>
> Don't forget the efficiency criterion.  One reason for going with fsinfo(2) is
> that scanning /proc/mounts when there are a lot of mounts in the system is
> slow (not to mention the global lock that is held during the read).
>
> Now, going with sysfs files on top of procfs links might avoid the global
> lock, and you can avoid rereading the options string if you export a change
> notification, but you're going to end up injecting a whole lot of pathwalk
> latency into the system.

Completely irrelevant.  Cached lookup is so much optimized, that you
won't be able to see any of it.

No, I don't think this is going to be a performance issue at all, but
if anything we could introduce a syscall

  ssize_t readfile(int dfd, const char *path, char *buf, size_t
bufsize, int flags);

that is basically the equivalent of open + read + close, or even a
vectored variant that reads multiple files.  But that's off topic
again, since I don't think there's going to be any performance issue
even with plain I/O syscalls.

>
> On top of that, it isn't going to help with the case that I'm working towards
> implementing where a container manager can monitor for mounts taking place
> inside the container and supervise them.  What I'm proposing is that during
> the action phase (eg. FSCONFIG_CMD_CREATE), fsconfig() would hand an fd
> referring to the context under construction to the manager, which would then
> be able to call fsinfo() to query it and fsconfig() to adjust it, reject it or
> permit it.  Something like:
>
>         fd = receive_context_to_supervise();
>         struct fsinfo_params params = {
>                 .flags          = FSINFO_FLAGS_QUERY_FSCONTEXT,
>                 .request        = FSINFO_ATTR_SB_OPTIONS,
>         };
>         fsinfo(fd, NULL, &params, sizeof(params), buffer, sizeof(buffer));
>         supervise_parameters(buffer);
>         fsconfig(fd, FSCONFIG_SET_FLAG, "hard", NULL, 0);
>         fsconfig(fd, FSCONFIG_SET_STRING, "vers", "4.2", 0);
>         fsconfig(fd, FSCONFIG_CMD_SUPERVISE_CREATE, NULL, NULL, 0);
>         struct fsinfo_params params = {
>                 .flags          = FSINFO_FLAGS_QUERY_FSCONTEXT,
>                 .request        = FSINFO_ATTR_SB_NOTIFICATIONS,
>         };
>         struct fsinfo_sb_notifications sbnotify;
>         fsinfo(fd, NULL, &params, sizeof(params), &sbnotify, sizeof(sbnotify));
>         watch_super(fd, "", AT_EMPTY_PATH, watch_fd, 0x03);
>         fsconfig(fd, FSCONFIG_CMD_SUPERVISE_PERMIT, NULL, NULL, 0);
>         close(fd);
>
> However, the supervised mount may be happening in a completely different set
> of namespaces, in which case the supervisor presumably wouldn't be able to see
> the links in procfs and the relevant portions of sysfs.

It would be a "jump" link to the otherwise invisible directory.

Thanks,
Miklos