Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE

[Vlastimil Babka]

On 10/24/22 17:06, Matthew Wilcox wrote:
> 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.

AFAIU (which doesn't mean much, TBH :)) folio_set_slab() makes the setting
of the flag protected against other flags set operations so our setting is
not lost in a non-atomic RMW. But as we are the only one who can be setting
any page/folio flag here (isolate_movable_page() for sure doesn't), we don't
need it for that kind of atomicity for page/folio flags field.

And, simply changing it to folio_set_slab() would not add the sufficient
smp_wmb() semantics to order the flags write visibility against a later
write to the struct slab field that overlaps page->mapping. Only some atomic
operations have that implicit barrier, (per
Documentation/memory-barriers.txt and Documentation/atomic_bitops.txt) and
set_bit() is not one of those. So we'd still need a smp_mb__after_atomic()
AFAIU and at that point, doing the above seems less obscure to me.

(Of course if we had the reason to use folio_set_slab() for its own atomic
guarantee, then smp_mb__after_atomic() instead of smp_wmb() would be better
as on some architectures it would make the barrier no-op).