Re: [External] Re: [PATCH v9 04/11] mm/hugetlb: Defer freeing of HugeTLB pages

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Dec 17, 2020 at 7:48 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 12/13/20 7:45 AM, 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 use GFP_ATOMIC to allocate the vmemmap pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> It is unfortunate we need to add this complexitty, but I can not think
> of another way.  One small comment (no required change) below.
>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

Thank you.

>
> > ---
> >  mm/hugetlb.c         | 77 ++++++++++++++++++++++++++++++++++++++++++++++++----
> >  mm/hugetlb_vmemmap.c | 12 --------
> >  mm/hugetlb_vmemmap.h | 17 ++++++++++++
> >  3 files changed, 88 insertions(+), 18 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 140135fc8113..0ff9b90e524f 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1292,15 +1292,76 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> >                                               unsigned int order) { }
> >  #endif
> >
> > -static void update_and_free_page(struct hstate *h, struct page *page)
> > +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.
> > + *
> > + * update_hpage_vmemmap_workfn() locklessly retrieves the linked list of
> > + * pages to be freed and frees them one-by-one. As the page->mapping pointer
> > + * is going to be cleared in update_hpage_vmemmap_workfn() anyway, it is
> > + * reused as the llist_node structure of a lockless linked list of huge
> > + * pages to be freed.
> > + */
> > +static LLIST_HEAD(hpage_update_freelist);
> > +
> > +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);
> > +
> > +static inline void __update_and_free_page(struct hstate *h, struct page *page)
> > +{
> > +     /* No need to allocate vmemmap pages */
> > +     if (!free_vmemmap_pages_per_hpage(h)) {
> > +             __free_hugepage(h, page);
> > +             return;
> > +     }
> > +
> > +     /*
> > +      * Defer freeing to avoid using GFP_ATOMIC to allocate vmemmap
> > +      * pages.
> > +      *
> > +      * Only call schedule_work() if hpage_update_freelist is previously
> > +      * empty. Otherwise, schedule_work() had been called but the workfn
> > +      * hasn't retrieved the list yet.
> > +      */
> > +     if (llist_add((struct llist_node *)&page->mapping,
> > +                   &hpage_update_freelist))
> > +             schedule_work(&hpage_update_work);
> > +}
> > +
> > +static void update_and_free_page(struct hstate *h, struct page *page)
> > +{
> >       if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> >               return;
> >
> >       h->nr_huge_pages--;
> >       h->nr_huge_pages_node[page_to_nid(page)]--;
> > +
> > +     __update_and_free_page(h, page);
> > +}
> > +
> > +static void __free_hugepage(struct hstate *h, struct page *page)
> > +{
> > +     int i;
> > +
>
> Can we add a comment here saying that this is where the call to allocate
> vmemmmap pages will be inserted in a later patch.  Such a comment would
> help a bit to understand the restructuring of the code.

OK. Will do. Thanks.

>
> --
> Mike Kravetz
>
> >       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 +1374,17 @@ 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
> > +              * contex do not hold the hugetlb_lock), because we might block
> > +              * in free_gigantic_page().
> >                */
> > -             spin_unlock(&hugetlb_lock);
> > +             if (!free_vmemmap_pages_per_hpage(h))
> > +                     spin_unlock(&hugetlb_lock);
> >               destroy_compound_gigantic_page(page, huge_page_order(h));
> >               free_gigantic_page(page, huge_page_order(h));
> > -             spin_lock(&hugetlb_lock);
> > +             if (!free_vmemmap_pages_per_hpage(h))
> > +                     spin_lock(&hugetlb_lock);
> >       } else {
> >               __free_pages(page, huge_page_order(h));
> >       }



-- 
Yours,
Muchun