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Date:   Mon, 2 Mar 2020 10:38:50 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     David Howells <dhowells@redhat.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        viro <viro@zeniv.linux.org.uk>, Ian Kent <raven@themaw.net>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver
 #17]
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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@redhat.com> 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