linux-ext4 - Re: [PATCH v4 07/17] fs/dcache.c: add shrink_dcache

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Date:   Wed, 3 Apr 2019 19:34:12 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fscrypt@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, linux-api@...r.kernel.org,
        linux-crypto@...r.kernel.org, keyrings@...r.kernel.org,
        Paul Crowley <paulcrowley@...gle.com>,
        Satya Tangirala <satyat@...gle.com>
Subject: Re: [PATCH v4 07/17] fs/dcache.c: add shrink_dcache_inode()

On Tue, Apr 02, 2019 at 08:45:50AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> When a filesystem encryption key is removed, we need all files which had
> been "unlocked" (had ->i_crypt_info set up) with it to appear "locked"
> again.  This is most easily done by evicting the inodes.  This can
> currently be done using 'echo 2 > /proc/sys/vm/drop_caches'; however,
> that is overkill and not usable by non-root users.
> 
> To evict just the needed inodes we also need the ability to evict those
> inodes' dentries, since an inode is pinned by its dentries.  Therefore,
> add a function shrink_dcache_inode() which iterates through an inode's
> dentries and evicts any unused ones as well as any unused descendants
> (since there may be negative dentries pinning the inode's dentries).

Huh?

> + * Evict all unused aliases of the specified inode from the dcache.  This is
> + * intended to be used when trying to evict a specific inode, since inodes are
> + * pinned by their dentries.  We also have to descend to ->d_subdirs for each
> + * alias, since aliases may be pinned by negative child dentries.
> + */
> +void shrink_dcache_inode(struct inode *inode)
> +{
> +	for (;;) {
> +		struct select_data data;
> +		struct dentry *dentry;
> +
> +		INIT_LIST_HEAD(&data.dispose);
> +		data.start = NULL;
> +		data.found = 0;
> +
> +		spin_lock(&inode->i_lock);
> +		hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias)
> +			d_walk(dentry, &data, select_collect);
> +		spin_unlock(&inode->i_lock);
> +
> +		if (!data.found)
> +			break;
> +
> +		shrink_dentry_list(&data.dispose);
> +		cond_resched();

This is... odd.  What's wrong with
	if (S_ISDIR(inode->i_mode)) {
		dentry = d_find_any_alias(inode);
		if (dentry) {
			shrink_dcache_parent(dentry);
			dput(dentry);
		}
	}
	d_prune_aliases(inode);
instead of that thing?