On Wed, Jun 30, 2021 at 02:45:33PM +0200, Michal Hocko wrote:

On Wed 30-06-21 13:31:17, Matthew Wilcox wrote:

quoted

On Wed, Jun 30, 2021 at 02:20:38PM +0200, Michal Hocko wrote:

quoted

On Wed 30-06-21 12:22:48, Matthew Wilcox wrote:

quoted

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.

The scenario I'm thinking about is a situation where we have gigabytes
of memory in the page cache in 16k-64k chunks and we'll see
FileHugePages: 5219348 kB
FilePmdMapped:       0 kB

which might cause the slightly-too-clever user to think there's a
problem.