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: <ea6e7b10-c514-8320-05c2-4a18391a7079@suse.cz>
Date:   Thu, 29 Sep 2022 15:01:08 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.3.0
Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE
Content-Language: en-US
To:     David Laight <David.Laight@ACULAB.COM>,
        'Hugh Dickins' <hughd@google.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.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>
 <9368c760191448bba0e062709cc7c799@AcuMS.aculab.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <9368c760191448bba0e062709cc7c799@AcuMS.aculab.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 9/29/22 13:53, David Laight wrote:
>> -static void rcu_free_slab(struct rcu_head *h)
>> +/*
>> + * rcu_free_slab() must be __aligned(4) because its address is saved
>> + * in the rcu_head field, which coincides with page->mapping, which
>> + * causes trouble if compaction mistakes it for PAGE_MAPPING_MOVABLE.
>> + */
>> +__aligned(4) static void rcu_free_slab(struct rcu_head *h)
>>  {
>>  	struct slab *slab = container_of(h, struct slab, rcu_head);
>> 
> 
> Isn't that going to cause grief with options that align
> functions on 16/32byte boundaries when adding space for
> 'other stuff'?

How is that done exactly? Also having higher alignment (16/32) is not in
conflict with asking for 4?

> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>