On Mon, Mar 02, 2020 at 10:09:51AM +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. That is correct, they should not be pinned, that is what kernfs handles and why we can handle 30k virtual block devices on a 31bit s390 instance :) So you shouldn't have to worry about memory for sysfs. There are loads of other reasons probably not to use sysfs for this instead :) thanks, greg k-h
