Re: memcg memory accounting in vmalloc is broken

Mel Gorman
2021-10-07T10:20:49Z

On Thu, Oct 07, 2021 at 11:50:44AM +0300, Vasily Averin wrote:
> On 10/7/21 11:16 AM, Michal Hocko wrote:
> > Cc Mel and Uladzislau
> >
> > On Thu 07-10-21 10:13:23, Michal Hocko wrote:
> >> On Thu 07-10-21 11:04:40, Vasily Averin wrote:
> >>> vmalloc was switched to __alloc_pages_bulk but it does not account the memory to memcg.
> >>>
> >>> Is it known issue perhaps?
> >>
> >> No, I think this was just overlooked. Definitely doesn't look
> >> intentional to me.
>
> I use following patch as a quick fix,
> it helps though it is far from ideal and can be optimized.

Thanks Vasily.

This papers over the problem but it could certainly be optimized. At
minimum;

1. Test (memcg_kmem_enabled() && (gfp & __GFP_ACCOUNT)) in the
function preamble and store the result in a bool
2. Avoid the temptation to batch the accounting because if the
accounting fails, there is no information on how many pages could be
allocated before the limits were hit. I guess you could pre-charge the
pages and uncharging the number of pages that failed to be allocated
but it should be a separate patch.
3. If an allocation fails due to memcg accounting, break
out of the loop because all remaining bulk allocations are
also likely to fail.

As it's not vmalloc's fault, I would suggest the patch
have
Fixes: 387ba26fb1cb ("mm/page_alloc: add a bulk page allocator")
and
Cc: <stable@vger.kernel.org>

Note the Cc should just be in the patch and not mailed directly to
stable@ as it'll simply trigger a form letter about the patch having to
be merged to mainline first.

--
Mel Gorman
SUSE Labs