Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

Matthew Wilcox
2022-10-24T19:21:20Z

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.