Re: [External] Re: [PATCH] mm: memcontrol: fix kernel stack account

[Muchun Song]

On Tue, Mar 2, 2021 at 5:34 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 02-03-21 17:23:42, Muchun Song wrote:
> > On Tue, Mar 2, 2021 at 4:44 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Tue 02-03-21 15:37:33, Muchun Song wrote:
> > > > The alloc_thread_stack_node() cannot guarantee that allocated stack pages
> > > > are in the same node when CONFIG_VMAP_STACK. Because we do not specify
> > > > __GFP_THISNODE to __vmalloc_node_range(). Fix it by caling
> > > > mod_lruvec_page_state() for each page one by one.
> > >
> > > What is the actual problem you are trying to address by this patch?
> > > 991e7673859e has deliberately dropped the per page accounting. Can you
> > > explain why that was incorrect? There surely is some imprecision
> > > involved but does it matter and is it even observable?
> >
> > When I read the code of account_kernel_stack(), I see a comment that
> > says "All stack pages are in the same node". I am confused about this.
> > IIUC, there is no guarantee about this. Right?
>
> Yes there is no guarantee indeed. Please always make sure to describe
> the underlying reasoning for the patch. Subject of this patch refers to
> a fix without explaining the actual problem. If a change is motivated by
> code reading then make it explicit. Also if you are refering to a
> different commit by Fixes: tag then it would be really helpful to
> explicitly mention why that commit is incorrect or cause a visible
> problems.

Got it. Thanks for your teaching.

>
> > Yeah, imprecision may
> > not be a problem. But if this is what we did deliberately, I think that
> > it is better to add a comment there. Thanks.
>
> Yes the comment is quite confusing. I suspect it meant to say
>         /* All stack pages are accounted to the same node */
>
> --
> Michal Hocko
> SUSE Labs