linux-ext4 - Re: [PATCH 1/3] fs/dcache: Add d_clear_dir_neg

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Message-ID: <20210328150715.GA33249@casper.infradead.org>
Date:   Sun, 28 Mar 2021 16:07:15 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     André Almeida <andrealmeid@...labora.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        krisman@...labora.com, kernel@...labora.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        Daniel Rosenberg <drosen@...gle.com>,
        Chao Yu <yuchao0@...wei.com>
Subject: Re: [PATCH 1/3] fs/dcache: Add d_clear_dir_neg_dentries()

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.