Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <32fe01dc-dda1-2154-af8e-7e9f64d6f811@suse.cz>
Date:   Tue, 25 Oct 2022 11:17:39 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.3.3
Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE
Content-Language: en-US
To:     Hugh Dickins <hughd@google.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        rcu@vger.kernel.org, David Laight <David.Laight@ACULAB.COM>,
        Kees Cook <keescook@chromium.org>
References: <ea96c78c-e1dc-1364-e91-51909f82388b@google.com>
 <YzPgTtFzpKEfwPbK@hyeyoo> <YzRQvoVsnJzsauwb@google.com>
 <35502bdd-1a78-dea1-6ac3-6ff1bcc073fa@suse.cz>
 <ff905c1e-5eb3-eaf8-46de-38f189c0b7a5@google.com>
 <de71b83a-c82c-4785-ef5a-3db4f17bbc8d@suse.cz>
 <bcecece-f7ce-221d-1674-da3d5ab3fef@google.com> <YzkmErHFyYW3awRn@hyeyoo>
 <YzsVM8eToHUeTP75@casper.infradead.org>
 <7dddca4c-bc36-2cf0-de1c-a770bef9e1b7@suse.cz>
 <904cc831-eb16-b1e5-10e5-9e7a171ef83@google.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <904cc831-eb16-b1e5-10e5-9e7a171ef83@google.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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