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.
