linux-kernel - Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR

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Message-ID: <YzRjR8r/dzU17Sw2@hyeyoo>
Date:   Thu, 29 Sep 2022 00:09:05 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

On Wed, Sep 28, 2022 at 01:48:46PM +0000, Joel Fernandes wrote:
> 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). :-)

Yeah, better to avoid that.

> > 2. make __PageMovable() to use true page flag, with approach [2])
> 
> What are the drawbacks of making it a true flag?

We don't have free bit in page flags unless we free one ;)

> 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,
Hyeonggon