Re: memcg memory accounting in vmalloc is broken

[Vlastimil Babka]

On 10/7/21 12:20, Mel Gorman wrote:
> 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

memcg_kmem_enabled() is a static key based check so caching that defeats its
purpose and changes it to a normal branch. That leaves gfp & __GFP_ACCOUNT,
which may perhaps still benefit from such caching.

> 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@xxxxxxxxxxxxxxx>
> 
> 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.
>