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

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Message-ID: <ea6e7b10-c514-8320-05c2-4a18391a7079@suse.cz>
Date:   Thu, 29 Sep 2022 15:01:08 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Laight <David.Laight@...LAB.COM>,
        'Hugh Dickins' <hughd@...gle.com>
Cc:     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-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

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
> 
> -
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