On Fri, Oct 01, 2021 at 10:52:10AM -0700, Mike Kravetz wrote:
> Demote page functionality will split a huge page into a number of huge
> pages of a smaller size. For example, on x86 a 1GB huge page can be
> demoted into 512 2M huge pages. Demotion is done 'in place' by simply
> splitting the huge page.
>
> Added '*_for_demote' wrappers for remove_hugetlb_page,
> destroy_compound_gigantic_page and prep_compound_gigantic_page for use
> by demote code.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
> mm/hugetlb.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 74 insertions(+), 8 deletions(-)
>
...
> +static int demote_free_huge_page(struct hstate *h, struct page *page)
> +{
> + int i, nid = page_to_nid(page);
> + struct hstate *target_hstate;
> + int rc = 0;
> +
> + target_hstate = size_to_hstate(PAGE_SIZE << h->demote_order);
> +
> + remove_hugetlb_page_for_demote(h, page, false);
> + spin_unlock_irq(&hugetlb_lock);
> +
> + rc = alloc_huge_page_vmemmap(h, page);
> + if (rc) {
> + /* Allocation of vmemmmap failed, we can not demote page */
> + spin_lock_irq(&hugetlb_lock);
> + set_page_refcounted(page);
> + add_hugetlb_page(h, page, false);
> + return rc;
> + }
Question: You keep the original error code returned from alloc_huge_page_vmemmap()
here, but then you lose it on demote_pool_huge_page() when doing the
!demote_free_huge_page. Would not make more sense to keep it all the way down to
demote_store() in case you want to return the actual error code?
> +
> + /*
> + * Use destroy_compound_gigantic_page_for_demote for all huge page
> + * sizes as it will not ref count pages.
> + */
> + destroy_compound_gigantic_page_for_demote(page, huge_page_order(h));
It seems that for now we only allow gigantic pages to be demoted, but
destroy_compound_gigantic_page_for_demote feels kind of wrong, even
if it is only a wrapper that ends up calling _*gigantic_ functions.
We want a routine that destroy a hugetlb to be demoted into smaller hugetlb
pages, so the name gigantic makes little sense to appear in my opinion.
> static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
> __must_hold(&hugetlb_lock)
> {
> + int nr_nodes, node;
> + struct page *page;
> int rc = 0;
>
> lockdep_assert_held(&hugetlb_lock);
> @@ -3313,9 +3377,15 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
> if (!h->demote_order)
> return rc;
>
> - /*
> - * TODO - demote fucntionality will be added in subsequent patch
> - */
> + for_each_node_mask_to_free(h, nr_nodes, node, nodes_allowed) {
> + if (!list_empty(&h->hugepage_freelists[node])) {
> + page = list_entry(h->hugepage_freelists[node].next,
> + struct page, lru);
> + rc = !demote_free_huge_page(h, page);
I kinda dislike this as I pointed out.
--
Oscar Salvador
SUSE Labs
