linux-kernel - Re: [fuse-devel] Kernel panic under load

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Date:   Fri, 9 Sep 2016 15:42:39 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Antonio SJ Musumeci <trapexit@...wn.link>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [fuse-devel] Kernel panic under load

Hi Miklos,

On Fri, Sep 09, 2016 at 04:32:49PM +0200, Miklos Szeredi wrote:
> On Fri, Sep 9, 2016 at 3:34 PM, Antonio SJ Musumeci <trapexit@...wn.link> wrote:
> > https://gist.github.com/bauruine/3bc00075c4d0b5b3353071d208ded30f
> > https://github.com/trapexit/mergerfs/issues/295
> >
> > I've some users which are having issues with my filesystem where the
> > system's load increases and then the kernel panics.
> >
> > Has anyone seen this before?
> 
> Quite possibly this is caused by fuse, but the BUG is deep in mm
> territory and I have zero clue about what it means.
> 
> Hannes,  can you please look a the above crash in mm/workingset.c?

The MM maintains a reclaimable list of page cache tree nodes that have
gone empty (all pages evicted) except for the shadow entries reclaimed
pages leave behind. When faulting a regular page back into such a node
the code in page_cache_tree_insert() removes it from the list again:

		workingset_node_pages_inc(node);
		/*
		 * Don't track node that contains actual pages.
		 *
		 * Avoid acquiring the list_lru lock if already
		 * untracked.  The list_empty() test is safe as
		 * node->private_list is protected by
		 * mapping->tree_lock.
		 */
		if (!list_empty(&node->private_list))
			list_lru_del(&workingset_shadow_nodes,
				     &node->private_list);

The BUG_ON() triggers when we later walk the reclaimable list and find
a radix tree node that has actual pages in it. This could happen when
pages are inserted into a mapping without using add_to_page_cache and
related functions. Does that maybe ring a bell?