Re: [PATCH 1/3] fs/dcache: Add d_clear_dir_neg_dentries()

[André Almeida]

Às 12:07 de 28/03/21, Matthew Wilcox escreveu:
> On Sun, Mar 28, 2021 at 11:43:54AM -0300, André Almeida wrote:
> > +/**
> > + * d_clear_dir_neg_dentries - Remove negative dentries in an inode
> > + * @dir: Directory to clear negative dentries
> > + *
> > + * For directories with negative dentries that are becoming case-insensitive
> > + * dirs, we need to remove all those negative dentries, otherwise they will
> > + * become dangling dentries. During the creation of a new file, if a d_hash
> > + * collision happens and the names match in a case-insensitive, the name of
> > + * the file will be the name defined at the negative dentry, that can be
> > + * different from the specified by the user. To prevent this from happening, we
> > + * need to remove all dentries in a directory. Given that the directory must be
> > + * empty before we call this function we are sure that all dentries there will
> > + * be negative.
> > + */
>
> This is quite the landmine of a function.  It _assumes_ that the directory
> is empty, and clears all dentries in it.
>
> > +void d_clear_dir_neg_dentries(struct inode *dir)
> > +{
> > +	struct dentry *alias, *dentry;
> > +
> > +	hlist_for_each_entry(alias, &dir->i_dentry, d_u.d_alias) {
> > +		list_for_each_entry(dentry, &alias->d_subdirs, d_child) {
> > +			d_drop(dentry);
> > +			dput(dentry);
> > +		}
>
> I would be happier if it included a check for negativity.  d_is_negative()
> or maybe this newfangled d_really_is_negative() (i haven't stayed up
> to speed on the precise difference between the two)

Makes sense. And given that this only makes sense if the directory is 
empty, if it founds a non-negative dentry, it should return some error 
right?

> > +	}
> > +}
> > +EXPORT_SYMBOL(d_clear_dir_neg_dentries);
>
> I'd rather see this _GPL for such an internal thing.