On 9/28/22 19:50, Hugh Dickins wrote: > On Wed, 28 Sep 2022, Vlastimil Babka wrote: >> On 9/28/22 15:48, Joel Fernandes wrote: >> > On Wed, Sep 28, 2022 at 02:49:02PM +0900, Hyeonggon Yoo wrote: >> >> On Tue, Sep 27, 2022 at 10:16:35PM -0700, Hugh Dickins wrote: >> >>> It's a bug in linux-next, but taking me too long to identify which >> >>> commit is "to blame", so let me throw it over to you without more >> >>> delay: I think __PageMovable() now needs to check !PageSlab(). >> >> When I tried that, the result wasn't really nice: >> >> https://lore.kernel.org/all/aec59f53-0e53-1736-5932-25407125d4d4@xxxxxxx/ >> >> And what if there's another conflicting page "type" later. Or the debugging >> variant of rcu_head in struct page itself. The __PageMovable() is just too >> fragile. > > I don't disagree (and don't really know all the things you're thinking > of in there). But if it's important to rescue this feature for 6.1, a > different approach may be the very simple patch below (I met a similar > issue with OPTIMIZE_FOR_SIZE in i915 a year ago, and just remembered). > > But you be the judge of it: (a) I do not know whether rcu_free_slab > is the only risky address ever stuffed into that field; and (b) I'm > clueless when it comes to those architectures (powerpc etc) where the > the address of a function is something different from the address of > the function (have I conveyed my cluelessness adequately?). Thanks a lot Hugh! That's a sufficiently small fix (compared to the other options) that I'm probably give it one last try. > Hugh > > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -1953,7 +1953,12 @@ static void __free_slab(struct kmem_cach > __free_pages(folio_page(folio, 0), order); > } > > -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); >
