Re: [External] Re: [PATCH v20 5/9] mm: hugetlb: defer freeing of HugeTLB pages

[Muchun Song <songmuchun@bytedance.com>]

On Sat, Apr 17, 2021 at 7:56 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 4/15/21 1:40 AM, Muchun Song wrote:
> > In the subsequent patch, we should allocate the vmemmap pages when
> > freeing a HugeTLB page. But update_and_free_page() can be called
> > under any context, so we cannot use GFP_KERNEL to allocate vmemmap
> > pages. However, we can defer the actual freeing in a kworker to
> > prevent from using GFP_ATOMIC to allocate the vmemmap pages.
>
> Thanks!  I knew we would need to introduce a kworker for this when I
> removed the kworker previously used in free_huge_page.

Yeah, but another choice is using GFP_ATOMIC to allocate vmemmap
pages when we are in an atomic context. If not atomic context, just
use GFP_KERNEL. In this case, we can drop kworker.

>
> > The __update_and_free_page() is where the call to allocate vmemmmap
> > pages will be inserted.
>
> This patch adds the functionality required for __update_and_free_page
> to potentially sleep and fail.  More questions will come up in the
> subsequent patch when code must deal with the failures.

Right. More questions are welcome.

>
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  mm/hugetlb.c         | 73 ++++++++++++++++++++++++++++++++++++++++++++++++----
> >  mm/hugetlb_vmemmap.c | 12 ---------
> >  mm/hugetlb_vmemmap.h | 17 ++++++++++++
> >  3 files changed, 85 insertions(+), 17 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 923d05e2806b..eeb8f5480170 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1376,7 +1376,7 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page,
> >       h->nr_huge_pages_node[nid]--;
> >  }
> >
> > -static void update_and_free_page(struct hstate *h, struct page *page)
> > +static void __update_and_free_page(struct hstate *h, struct page *page)
> >  {
> >       int i;
> >       struct page *subpage = page;
> > @@ -1399,12 +1399,73 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> >       }
> >  }
> >
> > +/*
> > + * As update_and_free_page() can be called under any context, so we cannot
> > + * use GFP_KERNEL to allocate vmemmap pages. However, we can defer the
> > + * actual freeing in a workqueue to prevent from using GFP_ATOMIC to allocate
> > + * the vmemmap pages.
> > + *
> > + * free_hpage_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 free_hpage_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_freelist);
> > +
> > +static void free_hpage_workfn(struct work_struct *work)
> > +{
> > +     struct llist_node *node;
> > +
> > +     node = llist_del_all(&hpage_freelist);
> > +
> > +     while (node) {
> > +             struct page *page;
> > +             struct hstate *h;
> > +
> > +             page = container_of((struct address_space **)node,
> > +                                  struct page, mapping);
> > +             node = node->next;
> > +             page->mapping = NULL;
> > +             h = page_hstate(page);
>
> The VM_BUG_ON_PAGE(!PageHuge(page), page) in page_hstate is going to
> trigger because a previous call to remove_hugetlb_page() will
> set_compound_page_dtor(page, NULL_COMPOUND_DTOR)

Sorry, I did not realise that. Thanks for your reminder.

>
> Note how h(hstate) is grabbed before calling update_and_free_page in
> existing code.
>
> We could potentially drop the !PageHuge(page) in page_hstate.  Or,
> perhaps just use 'size_to_hstate(page_size(page))' in free_hpage_workfn.

I prefer not to change the behavior of page_hstate(). So I
should use 'size_to_hstate(page_size(page))' directly.

Thanks Mike.


> --
> Mike Kravetz