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?
