Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

[Vlastimil Babka]

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);
>