Re: [PATCH] kernfs: attach uuid for every kernfs and report it in fsid

[Greg Kroah-Hartman]

On Mon, Jul 10, 2023 at 02:21:10PM -0700, Ivan Babrou wrote:
> On Mon, Jul 10, 2023 at 12:40 PM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Jul 10, 2023 at 11:33:38AM -0700, Ivan Babrou wrote:
> > > The following two commits added the same thing for tmpfs:
> > >
> > > * commit 2b4db79618ad ("tmpfs: generate random sb->s_uuid")
> > > * commit 59cda49ecf6c ("shmem: allow reporting fanotify events with file handles on tmpfs")
> > >
> > > Having fsid allows using fanotify, which is especially handy for cgroups,
> > > where one might be interested in knowing when they are created or removed.
> > >
> > > Signed-off-by: Ivan Babrou <ivan@xxxxxxxxxxxxxx>
> > > ---
> > >  fs/kernfs/mount.c | 13 ++++++++++++-
> > >  1 file changed, 12 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
> > > index d49606accb07..930026842359 100644
> > > --- a/fs/kernfs/mount.c
> > > +++ b/fs/kernfs/mount.c
> > > @@ -16,6 +16,8 @@
> > >  #include <linux/namei.h>
> > >  #include <linux/seq_file.h>
> > >  #include <linux/exportfs.h>
> > > +#include <linux/uuid.h>
> > > +#include <linux/statfs.h>
> > >
> > >  #include "kernfs-internal.h"
> > >
> > > @@ -45,8 +47,15 @@ static int kernfs_sop_show_path(struct seq_file *sf, struct dentry *dentry)
> > >       return 0;
> > >  }
> > >
> > > +int kernfs_statfs(struct dentry *dentry, struct kstatfs *buf)
> > > +{
> > > +     simple_statfs(dentry, buf);
> > > +     buf->f_fsid = uuid_to_fsid(dentry->d_sb->s_uuid.b);
> > > +     return 0;
> > > +}
> > > +
> > >  const struct super_operations kernfs_sops = {
> > > -     .statfs         = simple_statfs,
> > > +     .statfs         = kernfs_statfs,
> > >       .drop_inode     = generic_delete_inode,
> > >       .evict_inode    = kernfs_evict_inode,
> > >
> > > @@ -351,6 +360,8 @@ int kernfs_get_tree(struct fs_context *fc)
> > >               }
> > >               sb->s_flags |= SB_ACTIVE;
> > >
> > > +             uuid_gen(&sb->s_uuid);
> >
> > Since kernfs has as lot of nodes (like hundreds of thousands if not more
> > at times, being created at boot time), did you just slow down creating
> > them all, and increase the memory usage in a measurable way?
> 
> This is just for the superblock, not every inode. The memory increase
> is one UUID per kernfs instance (there are maybe 10 of them on a basic
> system), which is trivial. Same goes for CPU usage.

Ah, ok, my fault, thanks for clearing that up.

thanks,

greg k-h