Re: [PATCH RFC 1/6] dcache: sweep cached negative dentries to the end of list of siblings

[Al Viro]

On Thu, Jan 21, 2021 at 06:49:40PM +0530, Gautham Ananthakrishna wrote:
> From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> 
> For disk filesystems result of every negative lookup is cached, content of
> directories is usually cached too. Production of negative dentries isn't
> limited with disk speed. It's really easy to generate millions of them if
> system has enough memory. Negative dentries are linked into siblings list
> along with normal positive dentries. Some operations walks dcache tree but
> looks only for positive dentries: most important is fsnotify/inotify.
> 
> This patch moves negative dentries to the end of list at final dput() and
> marks with flag which tells that all following dentries are negative too.
> Reverse operation is required before instantiating negative dentry.

> +static void sweep_negative(struct dentry *dentry)
> +{
> +	struct dentry *parent;
> +
> +	if (!d_is_tail_negative(dentry)) {
> +		parent = lock_parent(dentry);
> +		if (!parent)
> +			return;
> +
> +		if (!d_count(dentry) && d_is_negative(dentry) &&
> +		    !d_is_tail_negative(dentry)) {
> +			dentry->d_flags |= DCACHE_TAIL_NEGATIVE;
> +			list_move_tail(&dentry->d_child, &parent->d_subdirs);
> +		}
> +
> +		spin_unlock(&parent->d_lock);
> +	}
> +}

Ugh...  So when dput() drives the refcount down to 0 you hit lock_parent()
and only then bother to check if the sucker had been negative in the first
place?

> @@ -1970,6 +2021,8 @@ void d_instantiate(struct dentry *entry, struct inode * inode)
>  {
>  	BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
>  	if (inode) {
> +		if (d_is_tail_negative(entry))
> +			recycle_negative(entry);
>  		security_d_instantiate(entry, inode);
>  		spin_lock(&inode->i_lock);
>  		__d_instantiate(entry, inode);

Wait a bloody minute.  What about d_instantiate_new() right next to it?