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

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Date:   Mon, 24 Oct 2022 16:06:29 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Hugh Dickins <hughd@...gle.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
Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

On Mon, Oct 24, 2022 at 04:35:04PM +0200, Vlastimil Babka wrote:
> I would like to have a working safe version in -next, even if we are able
> simplify it later thanks to frozen refcounts. I've made a formal patch of
> yours, but I'm still convinced the slab check needs to be more paranoid so
> it can't observe a false positive __folio_test_movable() while missing the
> folio_test_slab(), hence I added the barriers as in my previous attempt [1].
> Does that work for you and can I add your S-o-b?

Thanks for picking this back up.

> +++ b/mm/slab.c
> @@ -1370,6 +1370,8 @@ static struct slab *kmem_getpages(struct kmem_cache *cachep, gfp_t flags,
>  
>  	account_slab(slab, cachep->gfporder, cachep, flags);
>  	__folio_set_slab(folio);
> +	/* Make the flag visible before any changes to folio->mapping */
> +	smp_wmb();

So what's the point of using __folio_set_slab() only to call smp_wmb()
afterwards?  If we call folio_set_slab() instead, don't all the other
barriers go away?  (This is a genuine question; I am bad at this kind
of reasoning).  Obviously it would still need a comment.