Re: [External] Re: [PATCH v3 05/21] mm/hugetlb: Introduce pgtable allocation/freeing helpers

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Nov 11, 2020 at 8:47 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 11/8/20 6:10 AM, Muchun Song wrote:
> > On x86_64, vmemmap is always PMD mapped if the machine has hugepages
> > support and if we have 2MB contiguos pages and PMD aligned. If we want
> > to free the unused vmemmap pages, we have to split the huge pmd firstly.
> > So we should pre-allocate pgtable to split PMD to PTE.
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index a0007902fafb..5c7be2ee7e15 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1303,6 +1303,108 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> ...
> > +static inline void vmemmap_pgtable_init(struct page *page)
> > +{
> > +     page_huge_pte(page) = NULL;
> > +}
> > +
> > +static void vmemmap_pgtable_deposit(struct page *page, pgtable_t pgtable)
> > +{
> > +     /* FIFO */
> > +     if (!page_huge_pte(page))
> > +             INIT_LIST_HEAD(&pgtable->lru);
> > +     else
> > +             list_add(&pgtable->lru, &page_huge_pte(page)->lru);
> > +     page_huge_pte(page) = pgtable;
> > +}
> > +
> > +static pgtable_t vmemmap_pgtable_withdraw(struct page *page)
> > +{
> > +     pgtable_t pgtable;
> > +
> > +     /* FIFO */
> > +     pgtable = page_huge_pte(page);
> > +     page_huge_pte(page) = list_first_entry_or_null(&pgtable->lru,
> > +                                                    struct page, lru);
> > +     if (page_huge_pte(page))
> > +             list_del(&pgtable->lru);
> > +     return pgtable;
> > +}
> > +
> > +static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> > +{
> > +     int i;
> > +     pgtable_t pgtable;
> > +     unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> > +
> > +     if (!nr)
> > +             return 0;
> > +
> > +     vmemmap_pgtable_init(page);
> > +
> > +     for (i = 0; i < nr; i++) {
> > +             pte_t *pte_p;
> > +
> > +             pte_p = pte_alloc_one_kernel(&init_mm);
> > +             if (!pte_p)
> > +                     goto out;
> > +             vmemmap_pgtable_deposit(page, virt_to_page(pte_p));
> > +     }
> > +
> > +     return 0;
> > +out:
> > +     while (i-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> > +             pte_free_kernel(&init_mm, page_to_virt(pgtable));
> > +     return -ENOMEM;
> > +}
> > +
> > +static void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> > +{
> > +     pgtable_t pgtable;
> > +     unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> > +
> > +     if (!nr)
> > +             return;
> > +
> > +     pgtable = page_huge_pte(page);
> > +     if (!pgtable)
> > +             return;
> > +
> > +     while (nr-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> > +             pte_free_kernel(&init_mm, page_to_virt(pgtable));
> > +}
>
> I may be confused.
>
> In patch 9 of this series, the first call to vmemmap_pgtable_free() is made:
>
> > @@ -1645,6 +1799,10 @@ void free_huge_page(struct page *page)
> >
> >  static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> >  {
> > +     free_huge_page_vmemmap(h, page);
> > +     /* Must be called before the initialization of @page->lru */
> > +     vmemmap_pgtable_free(h, page);
> > +
> >       INIT_LIST_HEAD(&page->lru);
> >       set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> >       set_hugetlb_cgroup(page, NULL);
>
> When I saw that comment in previous patch series, I assumed page->lru was
> being used to store preallocated pages and pages to free.  However, unless

Yeah, you are right.

> I am reading the code incorrectly it does not appear page->lru (of the huge
> page) is being used for this purpose.  Is that correct?
>
> If it is correct, would using page->lru of the huge page make this code
> simpler?  I am just missing the reason why you are using
> page_huge_pte(page)->lru

For 1GB HugeTLB pages, we should pre-allocate more than one page
table. So I use a linked list. The page_huge_pte(page) is the list head.
Because the page->lru shares storage with page->pmd_huge_pte.

+     /* Must be called before the initialization of @page->lru */
+     vmemmap_pgtable_free(h, page);
+
       INIT_LIST_HEAD(&page->lru);

Here we initialize the lru. So the vmemmap_pgtable_free should
be called before this. It seems like that I should point out this "share"
in the comment.

Thanks.

>
> --
> Mike Kravetz



-- 
Yours,
Muchun