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

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Date:   Tue, 25 Oct 2022 11:17:39 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        rcu@...r.kernel.org, David Laight <David.Laight@...LAB.COM>,
        Kees Cook <keescook@...omium.org>
Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

On 10/25/22 06:19, Hugh Dickins wrote:
> On Mon, 24 Oct 2022, Vlastimil Babka wrote:

>> 
>> [1] https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@suse.cz/
> 
> Ignore me, don't let me distract if you're happy with Matthew's patch
> (I know little of PageMovable, and I haven't tried to understand it);
> but it did look to me more like 6.2 material,

Yes, it is targetted towards 6.2 now.

> and I was surprised that
> you dropped the simple align(4) approach for 6.1.
> 
> Because of Hyeonggon's rcu_leak_callback() observation?  That was a
> good catch, but turned out to be irrelevant, because it was only for
> an RCU debugging option, which would never be set up on a struct page
> (well, maybe it would in a dynamically-allocated-struct-page future).

It was mainly due to David's observation:

https://lore.kernel.org/all/c1c8920a56014a179dbb82d483f2ad28@AcuMS.aculab.com/

I did also conclude that align(4) indeed overrides anything set via
CONFIG_FUNCTION_ALIGNMENT (and passed by -falign-functions) in a way that a
globally configured larger alignment can be made smaller by this macro, and
it was all too risky and last minute to me, while Joel's series wasn't
targetted to 6.1 anyway so there was no urgency.

And yeah it looks like a gcc bug to me.

> Hugh