On Mon, 14 Jan 2019 09:53:43 +0100 Jan Kara <jack@xxxxxxx> wrote:
> When superblock has lots of inodes without any pagecache (like is the
> case for /proc), drop_pagecache_sb() will iterate through all of them
> without dropping sb->s_inode_list_lock which can lead to softlockups
> (one of our customers hit this).
>
> Fix the problem by going to the slow path and doing cond_resched() in
> case the process needs rescheduling.
>
> ...
>
> --- a/fs/drop_caches.c
> +++ b/fs/drop_caches.c
> @@ -21,8 +21,13 @@ static void drop_pagecache_sb(struct super_block *sb, void *unused)
> spin_lock(&sb->s_inode_list_lock);
> list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
> spin_lock(&inode->i_lock);
> + /*
> + * We must skip inodes in unusual state. We may also skip
> + * inodes without pages but we deliberately won't in case
> + * we need to reschedule to avoid softlockups.
> + */
> if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
> - (inode->i_mapping->nrpages == 0)) {
> + (inode->i_mapping->nrpages == 0 && !need_resched())) {
> spin_unlock(&inode->i_lock);
> continue;
> }
> @@ -30,6 +35,7 @@ static void drop_pagecache_sb(struct super_block *sb, void *unused)
> spin_unlock(&inode->i_lock);
> spin_unlock(&sb->s_inode_list_lock);
>
> + cond_resched();
> invalidate_mapping_pages(inode->i_mapping, 0, -1);
> iput(toput_inode);
> toput_inode = inode;
Are we sure there's no situation in which a large number of inodes can
be in the "unusual state", leading to the same issue?
