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

[Matthew Wilcox <willy@infradead.org>]

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)

> +	}
> +}
> +EXPORT_SYMBOL(d_clear_dir_neg_dentries);

I'd rather see this _GPL for such an internal thing.