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

[Waiman Long <longman@redhat.com>]

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@redhat.com>
> Good catch!
>
> Interesting that I haven't seen it on x86-64, but it's reproducible on Power.
>
> Acked-by: Roman Gushchin <guro@fb.com>
>
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