On Tue, Mar 3, 2020 at 6:28 AM Ian Kent <raven@xxxxxxxxxx> wrote: > > On Mon, 2020-03-02 at 10:09 +0100, Miklos Szeredi wrote: > > On Fri, Feb 28, 2020 at 5:36 PM David Howells <dhowells@xxxxxxxxxx> > > wrote: > > > sysfs also has some other disadvantages for this: > > > > > > (1) There's a potential chicken-and-egg problem in that you have > > > to create a > > > bunch of files and dirs in sysfs for every created mount and > > > superblock > > > (possibly excluding special ones like the socket mount) - but > > > this > > > includes sysfs itself. This might work - provided you create > > > sysfs > > > first. > > > > Sysfs architecture looks something like this (I hope Greg will > > correct > > me if I'm wrong): > > > > device driver -> kobj tree <- sysfs tree > > > > The kobj tree is created by the device driver, and the dentry tree is > > created on demand from the kobj tree. Lifetime of kobjs is bound to > > both the sysfs objects and the device but not the other way round. > > I.e. device can go away while the sysfs object is still being > > referenced, and sysfs can be freely mounted and unmounted > > independently of device initialization. > > > > So there's no ordering requirement between sysfs mounts and other > > mounts. I might be wrong on the details, since mounts are created > > very early in the boot process... > > > > > (2) sysfs is memory intensive. The directory structure has to be > > > backed by > > > dentries and inodes that linger as long as the referenced > > > object does > > > (procfs is more efficient in this regard for files that aren't > > > being > > > accessed) > > > > See above: I don't think dentries and inodes are pinned, only kobjs > > and their associated cruft. Which may be too heavy, depending on the > > details of the kobj tree. > > > > > (3) It gives people extra, indirect ways to pin mount objects and > > > superblocks. > > > > See above. > > > > > For the moment, fsinfo() gives you three ways of referring to a > > > filesystem > > > object: > > > > > > (a) Directly by path. > > > > A path is always representable by an O_PATH descriptor. > > > > > (b) By path associated with an fd. > > > > See my proposal about linking from /proc/$PID/fdmount/$FD -> > > /sys/devices/virtual/mounts/$MOUNT_ID. > > > > > (c) By mount ID (perm checked by working back up the tree). > > > > Check that perm on lookup of /sys/devices/virtual/mounts/$MOUNT_ID. > > The proc symlink would bypass the lookup check by directly jumping to > > the mountinfo dir. > > > > > but will need to add: > > > > > > (d) By fscontext fd (which is hard to find in sysfs). Indeed, the > > > superblock > > > may not even exist yet. > > > > Proc symlink would work for that too. > > There's mounts enumeration too, ordering is required to identify the > top (or bottom depending on terminology) with more than one mount on > a mount point. > > > > > If sysfs is too heavy, this could be proc or a completely new > > filesystem. The implementation is much less relevant at this stage > > of > > the discussion than the interface. > > Ha, proc with the seq file interface, that's already proved to not > work properly and looks difficult to fix. I'm doing a patch. Let's see how it fares in the face of all these preconceptions. Thanks, Miklos
