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Date:   Mon, 10 Jul 2023 21:41:02 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Ivan Babrou <ivan@cloudflare.com>
Cc:     linux-fsdevel@vger.kernel.org, kernel-team@cloudflare.com,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Tejun Heo <tj@kernel.org>, Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Amir Goldstein <amir73il@gmail.com>,
        Christoph Hellwig <hch@lst.de>, Jan Kara <jack@suse.cz>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH] kernfs: attach uuid for every kernfs and report it in
 fsid
Message-ID: <2023071046-paramount-climatic-31cb@gregkh>
References: <20230710183338.58531-1-ivan@cloudflare.com>
 <2023071039-negate-stalemate-6987@gregkh>
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On Mon, Jul 10, 2023 at 09:40:23PM +0200, Greg Kroah-Hartman 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@cloudflare.com>
> > ---
> >  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?
> 
> We were trying to slim things down, what userspace tools need this
> change?  Who is going to use it, and what for?
> 
> There were some benchmarks people were doing with booting large memory
> systems that you might want to reproduce here to verify that nothing is
> going to be harmed.

Oh wait, is this just a per-superblock thing?

confused,

greg k-h