On Mon, Dec 21, 2020 at 6:27 PM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Thu, Dec 17, 2020 at 08:12:56PM +0800, Muchun Song wrote:
> > In the subsequent patch, we will allocate the vmemmap pages when free
> > HugeTLB pages. But update_and_free_page() is called from a non-task
> > context(and hold hugetlb_lock), so we can defer the actual freeing in
> > a workqueue to prevent from using GFP_ATOMIC to allocate the vmemmap
> > pages.
>
> I think we would benefit from a more complete changelog, at least I had
> to stare at the code for a while in order to grasp what are we trying
> to do and the reasons behind.
OK. Will do.
>
> > +static void __free_hugepage(struct hstate *h, struct page *page);
> > +
> > +/*
> > + * As update_and_free_page() is be called from a non-task context(and hold
> > + * hugetlb_lock), we can defer the actual freeing in a workqueue to prevent
> > + * use GFP_ATOMIC to allocate a lot of vmemmap pages.
>
> The above implies that update_and_free_page() is __always__ called from a
> non-task context, but that is not always the case?
IIUC, here is always the case.
>
> > +static void update_hpage_vmemmap_workfn(struct work_struct *work)
> > {
> > - int i;
> > + struct llist_node *node;
> > + struct page *page;
> >
> > + node = llist_del_all(&hpage_update_freelist);
> > +
> > + while (node) {
> > + page = container_of((struct address_space **)node,
> > + struct page, mapping);
> > + node = node->next;
> > + page->mapping = NULL;
> > + __free_hugepage(page_hstate(page), page);
> > +
> > + cond_resched();
> > + }
> > +}
> > +static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);
>
> I wonder if this should be moved to hugetlb_vmemmap.c
Maybe I can do a try.
>
> > +/*
> > + * This is where the call to allocate vmemmmap pages will be inserted.
> > + */
>
> I think this should go in the changelog.
OK. Will do.
>
> > +static void __free_hugepage(struct hstate *h, struct page *page)
> > +{
> > + int i;
> > +
> > for (i = 0; i < pages_per_huge_page(h); i++) {
> > page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
> > 1 << PG_referenced | 1 << PG_dirty |
> > @@ -1313,13 +1377,17 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> > set_page_refcounted(page);
> > if (hstate_is_gigantic(h)) {
> > /*
> > - * Temporarily drop the hugetlb_lock, because
> > - * we might block in free_gigantic_page().
> > + * Temporarily drop the hugetlb_lock only when this type of
> > + * HugeTLB page does not support vmemmap optimization (which
> > + * context do not hold the hugetlb_lock), because we might
> > + * block in free_gigantic_page().
>
> "
> /*
> * Temporarily drop the hugetlb_lock, because we might block
> * in free_gigantic_page(). Only drop it in case the vmemmap
> * optimization is disabled, since that context does not hold
> * the lock.
> */
> " ?
Thanks a lot.
>
>
> Oscar Salvador
> SUSE L3
--
Yours,
Muchun
