Re: [PATCH] vfs: Avoid softlockups in drop_pagecache_sb()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 14 Jan 2019 09:53:43 +0100 Jan Kara <jack@suse.cz> 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?