Messages in this thread | | | From | Miklos Szeredi <> | Date | Tue, 31 Mar 2020 17:10:34 +0200 | Subject | Re: Upcoming: Notifications, FS notifications and fsinfo() |
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On Tue, Mar 31, 2020 at 2:25 PM Lennart Poettering <mzxreary@0pointer.de> wrote: > > On Di, 31.03.20 10:56, Miklos Szeredi (miklos@szeredi.hu) wrote: > > > On Tue, Mar 31, 2020 at 10:34 AM Karel Zak <kzak@redhat.com> wrote: > > > > > > On Tue, Mar 31, 2020 at 07:11:11AM +0200, Miklos Szeredi wrote: > > > > On Mon, Mar 30, 2020 at 11:17 PM Christian Brauner > > > > <christian.brauner@ubuntu.com> wrote: > > > > > > > > > Fwiw, putting down my kernel hat and speaking as someone who maintains > > > > > two container runtimes and various other low-level bits and pieces in > > > > > userspace who'd make heavy use of this stuff I would prefer the fd-based > > > > > fsinfo() approach especially in the light of across namespace > > > > > operations, querying all properties of a mount atomically all-at-once, > > > > > > > > fsinfo(2) doesn't meet the atomically all-at-once requirement. > > > > > > I guess your /proc based idea have exactly the same problem... > > > > Yes, that's exactly what I wanted to demonstrate: there's no > > fundamental difference between the two API's in this respect. > > > > > I see two possible ways: > > > > > > - after open("/mnt", O_PATH) create copy-on-write object in kernel to > > > represent mount node -- kernel will able to modify it, but userspace > > > will get unchanged data from the FD until to close() > > > > > > - improve fsinfo() to provide set (list) of the attributes by one call > > > > I think we are approaching this from the wrong end. Let's just > > ignore all of the proposed interfaces for now and only concentrate on > > what this will be used for. > > > > Start with a set of use cases by all interested parties. E.g. > > > > - systemd wants to keep track attached mounts in a namespace, as well > > as new detached mounts created by fsmount() > > > > - systemd need to keep information (such as parent, children, mount > > flags, fs options, etc) up to date on any change of topology or > > attributes. > > - We also have code that recursively remounts r/o or unmounts some > directory tree (with filters),
Recursive remount-ro is clear. What is not clear is whether you need to do this for hidden mounts (not possible from userspace without a way to disable mount following on path lookup). Would it make sense to add a kernel API for recursive setting of mount flags?
What exactly is this unmount with filters? Can you give examples?
> - We also have code that needs to check if /dev/ is plain tmpfs or > devtmpfs. We cannot use statfs for that, since in both cases > TMPFS_MAGIC is reported, hence we currently parse > /proc/self/mountinfo for that to find the fstype string there, which > is different for both cases.
Okay.
Thanks, Miklos
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