Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

Hyeonggon Yoo
2022-09-28T05:49:16Z

On Tue, Sep 27, 2022 at 10:16:35PM -0700, Hugh Dickins wrote:
> It's a bug in linux-next, but taking me too long to identify which
> commit is "to blame", so let me throw it over to you without more
> delay: I think __PageMovable() now needs to check !PageSlab().
>
> I had made a small experimental change somewhere, rebuilt and rebooted,
> was not surprised to crash once swapping and compaction came in,
> but was surprised to find the crash in isolate_movable_page(),
> called by compaction's isolate_migratepages_block().
>
> page->mapping was ffffffff811303aa, which qualifies as __PageMovable(),
> which expects struct movable_operations at page->mapping minus low bits.
> But ffffffff811303aa was the address of SLUB's rcu_free_slab(): I have
> CONFIG_CC_OPTIMIZE_FOR_SIZE=y, so function addresses may have low bits set.
>
> Over to you! Thanks,
> Hugh

Wow, didn't expect this.
Thank you for report!

That should be due to commit 65505d1f2338e7
("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head")
as now rcu_head can use some bits that shares with mapping.

Hmm IMO we have two choices...

1. simply drop the commit as it's only for debugging (RCU folks may not like [1])
2. make __PageMovable() to use true page flag, with approach [2])

[1] https://lore.kernel.org/all/85afd876-d8bb-0804-b2c5-48ed3055e702@joelfernandes.org/
[2] https://lore.kernel.org/linux-mm/20220919125708.276864-1-42.hyeyoo@gmail.com/

Thanks!

--
Thanks,
Hyeonggon