Re: [PATCH] mm, slab: Fix sign conversion problem in memcg_uncharge_slab()

[Roman Gushchin <guro@fb.com>]

On Sat, Jun 20, 2020 at 02:00:18PM -0700, Andrew Morton wrote:
> On Sat, 20 Jun 2020 14:47:19 -0400 Waiman Long <longman@redhat.com> wrote:
> 
> > It was found that running the LTP test on a PowerPC system could produce
> > erroneous values in /proc/meminfo, like:
> > 
> >   MemTotal:       531915072 kB
> >   MemFree:        507962176 kB
> >   MemAvailable:   1100020596352 kB
> > 
> > Using bisection, the problem is tracked down to commit 9c315e4d7d8c
> > ("mm: memcg/slab: cache page number in memcg_(un)charge_slab()").
> > 
> > In memcg_uncharge_slab() with a "int order" argument:
> > 
> >   unsigned int nr_pages = 1 << order;
> >     :
> >   mod_lruvec_state(lruvec, cache_vmstat_idx(s), -nr_pages);
> > 
> > The mod_lruvec_state() function will eventually call the
> > __mod_zone_page_state() which accepts a long argument.  Depending on
> > the compiler and how inlining is done, "-nr_pages" may be treated as
> > a negative number or a very large positive number. Apparently, it was
> > treated as a large positive number in that PowerPC system leading to
> > incorrect stat counts. This problem hasn't been seen in x86-64 yet,
> > perhaps the gcc compiler there has some slight difference in behavior.
> > 
> > It is fixed by making nr_pages a signed value. For consistency, a
> > similar change is applied to memcg_charge_slab() as well.
> 
> This is somewhat disturbing.
> 
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -348,7 +348,7 @@ static __always_inline int memcg_charge_slab(struct page *page,
> >  					     gfp_t gfp, int order,
> >  					     struct kmem_cache *s)
> >  {
> > -	unsigned int nr_pages = 1 << order;
> > +	int nr_pages = 1 << order;
> >  	struct mem_cgroup *memcg;
> >  	struct lruvec *lruvec;
> >  	int ret;
> > @@ -388,7 +388,7 @@ static __always_inline int memcg_charge_slab(struct page *page,
> >  static __always_inline void memcg_uncharge_slab(struct page *page, int order,
> >  						struct kmem_cache *s)
> >  {
> > -	unsigned int nr_pages = 1 << order;
> > +	int nr_pages = 1 << order;
> >  	struct mem_cgroup *memcg;
> >  	struct lruvec *lruvec;
> >  
> 
> I grabbed the patch, but Roman's "mm: memcg/slab: charge individual
> slab objects instead of pages"
> (http://lkml.kernel.org/r/20200608230654.828134-10-guro@fb.com) deletes
> both these functions.

It looks like Waiman's patch should be backported to stable.
So if you can queue it before my series, that would be nice.

> 
> It replaces the offending code with, afaict,
> 
> 
> static inline void memcg_slab_free_hook(struct kmem_cache *s, struct page *page,
> 					void *p)
> {
> 	struct obj_cgroup *objcg;
> 	unsigned int off;
> 
> 	if (!memcg_kmem_enabled() || is_root_cache(s))
> 		return;
> 
> 	off = obj_to_index(s, page, p);
> 	objcg = page_obj_cgroups(page)[off];
> 	page_obj_cgroups(page)[off] = NULL;
> 
> 	obj_cgroup_uncharge(objcg, obj_full_size(s));
> 	mod_objcg_state(objcg, page_pgdat(page), cache_vmstat_idx(s),
> >>>			-obj_full_size(s));
> 
> 	obj_cgroup_put(objcg);
> }
> 
> -obj_full_size() returns size_t so I guess that's OK.
> 
> 
> 
> Also
> 
> 
> static __always_inline void uncharge_slab_page(struct page *page, int order,
> 					       struct kmem_cache *s)
> {
> #ifdef CONFIG_MEMCG_KMEM
> 	if (memcg_kmem_enabled() && !is_root_cache(s)) {
> 		memcg_free_page_obj_cgroups(page);
> 		percpu_ref_put_many(&s->memcg_params.refcnt, 1 << order);
> 	}
> #endif
> 	mod_node_page_state(page_pgdat(page), cache_vmstat_idx(s),
> >>>			    -(PAGE_SIZE << order));
> }
> 
> PAGE_SIZE is unsigned long so I guess that's OK as well.
> 
> 
> Still, perhaps both could be improved.  Negating an unsigned scalar is
> a pretty ugly thing to do.
> 
> Am I wrong in thinking that all those mod_foo() functions need careful
> review?
> 

I'll take a look too.


Thanks!