Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

[Hugh Dickins <hughd@google.com>]

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@suse.cz/
> 
> 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?).

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);