linux-kernel - Re: BUG_ON() in workingset_node_shadows

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Message-ID: <CA+55aFzyXEwKAaX6zo-9DdXRjzAVP0Lwnr7gsAEm6Q8CGT+EFg@mail.gmail.com>
Date:   Tue, 4 Oct 2016 18:21:37 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Antonio SJ Musumeci <trapexit@...wn.link>,
        Miklos Szeredi <miklos@...redi.hu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: BUG_ON() in workingset_node_shadows_dec() triggers

On Tue, Oct 4, 2016 at 2:32 AM, Johannes Weiner <hannes@...xchg.org> wrote:
>
> In the workingset code, if we detect radix tree nodes in a state in
> which they shouldn't be on the shadow node LRU, we could simply warn,
> abort the reclaim process and leave them off the LRU. Something like
> the below patch.

I don't hate that patch, but I wonder why the counts get corrupted and
the workingset_node_shadows_dec() thing triggered in the first place.

So I do think that the BUG_ON()'s there in shadow_lru_isolate() should
be removed, but since they haven't triggered I worry more abut the one
that has.

I've tried to follow the counting, and I don't see any obvious bugs in
the counting per se.  I went as far as look where we even initialize
node->count.

Btw, whoever wrote that code liked the whole SLAB desctructor model a
lot too much. Initializing the fields as you free something is rather
silly from a cache use standpoint. You're just touching cachelines
that are almost guaranteed to be wasted. Why isn't that init code just
done at allocation time instead of in that radix_tree_node_rcu_free()
destructor?

But I couldn't see anything actively *buggy*, even if I think the code
is oddly structured.

So to debug that, I'd actually like to see something that adds a few
more warnings to try to catch *where* the count goes bad

For example, is it actually valid to free a radix_tree_node that has a
non-zero count? Shouldn't all the shadow entries have been removed?
The problem with the BUG_ON() at workingset_node_shadows_dec() time
isn't just that it killed the machine, it also doesn't actually give
very much information. The count has presumably been mis-done long
before..

                  Linus