Re: [PATCH v4 1/3] libfs: Add directory operations for stable offsets

[Christian Brauner <brauner@kernel.org>]

On Mon, Jun 26, 2023 at 11:44:15PM -0700, Christoph Hellwig wrote:
> > + * @dir: parent directory to be initialized
> > + *
> > + */
> > +void stable_offset_init(struct inode *dir)
> > +{
> > +	xa_init_flags(&dir->i_doff_map, XA_FLAGS_ALLOC1);
> > +	dir->i_next_offset = 0;
> > +}
> > +EXPORT_SYMBOL(stable_offset_init);
> 
> If this is exported I'd much prefer a EXPORT_SYMBOL_GPL.  But the only
> user so far is shmfs, which can't be modular anyway, so I think we can
> drop the exports.
> 
> > --- a/include/linux/dcache.h
> > +++ b/include/linux/dcache.h
> > @@ -96,6 +96,7 @@ struct dentry {
> >  	struct super_block *d_sb;	/* The root of the dentry tree */
> >  	unsigned long d_time;		/* used by d_revalidate */
> >  	void *d_fsdata;			/* fs-specific data */
> > +	u32 d_offset;			/* directory offset in parent */
> >  
> >  	union {
> >  		struct list_head d_lru;		/* LRU list */
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 133f0640fb24..3fc2c04ed8ff 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -719,6 +719,10 @@ struct inode {
> >  #endif
> >  
> >  	void			*i_private; /* fs or device private pointer */
> > +
> > +	/* simplefs stable directory offset tracking */
> > +	struct xarray		i_doff_map;
> > +	u32			i_next_offset;
> 
> We can't just increase the size of the dentry and inode for everyone
> for something that doesn't make any sense for normal file systems.
> This needs to move out into structures allocated by the file system
> and embedded into or used as the private dentry/inode data.

I agree. I prefer if this could be done on a per filesystem basis as
well. Especially since, this is currently only useful for a single
filesystem.

We've tried to be very conservative in increasing inode and dentry size
and we should continue with that.