Re: [External] Re: [PATCH v10 04/11] mm/hugetlb: Defer freeing of HugeTLB pages

[Muchun Song]

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