Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Sep 28, 2022 at 02:49:02PM +0900, Hyeonggon Yoo wrote:
> 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])

Yeah definitely don't like this option as patches are out that depend on
this (not yet merged though). :-)

> 2. make __PageMovable() to use true page flag, with approach [2])

What are the drawbacks of making it a true flag?

thanks,

 - Joel




> [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