Re: [PATCH v3 14/18] mm/memcg: Convert mem_cgroup_move_account() to use a folio

[Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>]

On Wed 30-06-21 13:31:17, Matthew Wilcox wrote:
> On Wed, Jun 30, 2021 at 02:20:38PM +0200, Michal Hocko wrote:
> > On Wed 30-06-21 12:22:48, Matthew Wilcox wrote:
> > > We need to decide what 'NR_ANON_THPS' means in a folio-based world where
> > > we have folios of all orders.  Does it count only the number of pages
> > > in folios >= HPAGE_PMD_SIZE?  Or does it count the number of pages in
> > > folios > PAGE_SIZE?
> > 
> > At this stage we only have PMD based, right? I believe it would be
> > simpler to stick with that at the moment and change that to a more
> > generic way along with other places which need updating.
> > 
> > Wrt. counters they do count pages so in this case this shouldn't be a
> > problem. But we do have counters for pmd mappings and that might need
> > some care.
> 
> Looking at how these are reported:
> 
>         show_val_kb(m, "AnonHugePages:  ",
>                     global_node_page_state(NR_ANON_THPS));
>         show_val_kb(m, "ShmemHugePages: ",
>                     global_node_page_state(NR_SHMEM_THPS));
>         show_val_kb(m, "ShmemPmdMapped: ",
>                     global_node_page_state(NR_SHMEM_PMDMAPPED));
>         show_val_kb(m, "FileHugePages:  ",
>                     global_node_page_state(NR_FILE_THPS));
>         show_val_kb(m, "FilePmdMapped:  ",
>                     global_node_page_state(NR_FILE_PMDMAPPED));
> 
> it specifically refers to 'HugePages', so I think we need to only
> count folios with order >= PMD_ORDER.

Why? The presented value is in kB. It gives us a cumulative number of
transparent large pages.  Sure breakdown to respective orders would be
impossible in general but the same would be the case if order > PMD_ORDER.

I am not really sure how useful that information is in practice but that
is a different story.

> I'll make that change to
> folio_transhuge() and use folio_transhuge() here.

Thanks!

-- 
Michal Hocko
SUSE Labs