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

[Waiman Long]

On 6/20/20 3:59 PM, Roman Gushchin wrote:
> On Sat, Jun 20, 2020 at 02:47:19PM -0400, Waiman Long 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.
> >
> > Fixes: 9c315e4d7d8c ("mm: memcg/slab: cache page number in memcg_(un)charge_slab()").
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Good catch!
>
> Interesting that I haven't seen it on x86-64, but it's reproducible on Power.
>
> Acked-by: Roman Gushchin <guro@xxxxxx>
I think it is probably related to the level of inlining that are being 
done. Besides, the interpretation of -nr_pages is ambiguous if nr_pages 
is an unsigned value and different compilers may handle it differently 
when sign extension is needed.

Cheers,
Longman