Date:   Fri, 30 Sep 2022 09:39:47 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        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 9/29/22 23:54, Hugh Dickins wrote:
> On Thu, 29 Sep 2022, Vlastimil Babka wrote:
>> On 9/28/22 19:50, Hugh Dickins wrote:
>> > On Wed, 28 Sep 2022, Vlastimil Babka wrote:
>> >> On 9/28/22 15:48, 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().
>> >> 
>> >> When I tried that, the result wasn't really nice:
>> >> 
>> >> https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@suse.cz/
>> >> 
>> >> And what if there's another conflicting page "type" later. Or the debugging
>> >> variant of rcu_head in struct page itself. The __PageMovable() is just too
>> >> fragile.
>> > 
>> > I don't disagree (and don't really know all the things you're thinking
>> > of in there).  But if it's important to rescue this feature for 6.1, a
>> > different approach may be the very simple patch below (I met a similar
>> > issue with OPTIMIZE_FOR_SIZE in i915 a year ago, and just remembered).
>> > 
>> > But you be the judge of it: (a) I do not know whether rcu_free_slab
>> > is the only risky address ever stuffed into that field; and (b) I'm
>> > clueless when it comes to those architectures (powerpc etc) where the
>> > the address of a function is something different from the address of
>> > the function (have I conveyed my cluelessness adequately?).
>> 
>> Thanks a lot Hugh! That's a sufficiently small fix (compared to the other
>> options) that I'm probably give it one last try.
> 
> I suddenly worried that you might be waiting on me for a Signed-off-by,
> which I couldn't give until I researched my reservations (a) and (b):
> but I'm pleased to see from your kernel.org tree that you've gone ahead
> and folded it in - thanks.

Yeah could have been more explicit about that, sorry. But made the whole
thing a very last merge so I can still drop it before the pull request.

> Regarding (a): great, you've found it too, mm/slab.c's kmem_rcu_free()
> looks like it needs the same __aligned(4) as mm/slub.c's rcu_free_slabi().

Right.

> Regarding (b): I booted the PowerMac G5 to take a look, and dredged up
> the relevant phrase "function descriptor" from depths of my memory: I
> was right to consider that case, but it's not a worry - the first field
> of a function descriptor structure (on all the architectures I found it)
> is the function address, so the function descriptor address would be
> aligned 4 or 8 anyway.

Thanks. I admit I wasn't that thorough, just consulted somebody internally :)

> Regarding "conflicting" alignment requests: yes, I agree with you,
> it would have to be a toolchain bug if when asked to align 2 and to
> align 4, it chose not to align 4.

Yeah. But I still would be less worried if another __aligned(X) function
existed in the tree already. Found only data. I assume the i915 thing wasn't
fixed like this in the tree? So if there are buggy toolchains or anything,
it will be us to discover them.

So I think we still should defuse the __PageMovable() mine somehow.

> So, no worries at my end now.
> Hugh

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