Re: [External] Re: [PATCH v2 05/19] mm/hugetlb: Introduce pgtable allocation/freeing helpers

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Nov 5, 2020 at 9:23 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Mon, Oct 26, 2020 at 10:51:00PM +0800, Muchun Song wrote:
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#define VMEMMAP_HPAGE_SHIFT                  PMD_SHIFT
> > +#define arch_vmemmap_support_huge_mapping()  boot_cpu_has(X86_FEATURE_PSE)
>
> I do not think you need this.
> We already have hugepages_supported().

Maybe some architectures support hugepage, but the vmemmap do not
use the hugepage map. In  this case, we need it. But I am not sure if it
exists in the real world. At least, x86 can reuse hugepages_supported.

>
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#ifndef arch_vmemmap_support_huge_mapping
> > +static inline bool arch_vmemmap_support_huge_mapping(void)
> > +{
> > +     return false;
> > +}
>
> Same as above
>
> >  static inline unsigned int nr_free_vmemmap(struct hstate *h)
> >  {
> >       return h->nr_free_vmemmap_pages;
> >  }
> >
> > +static inline unsigned int nr_vmemmap(struct hstate *h)
> > +{
> > +     return nr_free_vmemmap(h) + RESERVE_VMEMMAP_NR;
> > +}
> > +
> > +static inline unsigned long nr_vmemmap_size(struct hstate *h)
> > +{
> > +     return (unsigned long)nr_vmemmap(h) << PAGE_SHIFT;
> > +}
> > +
> > +static inline unsigned int nr_pgtable(struct hstate *h)
> > +{
> > +     unsigned long vmemmap_size = nr_vmemmap_size(h);
> > +
> > +     if (!arch_vmemmap_support_huge_mapping())
> > +             return 0;
> > +
> > +     /*
> > +      * No need pre-allocate page tabels when there is no vmemmap pages
> > +      * to free.
> > +      */
> > +     if (!nr_free_vmemmap(h))
> > +             return 0;
> > +
> > +     return ALIGN(vmemmap_size, VMEMMAP_HPAGE_SIZE) >> VMEMMAP_HPAGE_SHIFT;
> > +}
>
> IMHO, Mike's naming suggestion fit much better.

I will do that.

>
> > +static void vmemmap_pgtable_deposit(struct page *page, pte_t *pte_p)
> > +{
> > +     pgtable_t pgtable = virt_to_page(pte_p);
> > +
> > +     /* 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;
> > +}
>
> I think it would make more sense if this took a pgtable argument
> instead of a pte_t *.

Will do. Thanks for your suggestions.

>
> > +static pte_t *vmemmap_pgtable_withdraw(struct page *page)
> > +{
> > +     pgtable_t pgtable;
> > +
> > +     /* FIFO */
> > +     pgtable = page_huge_pte(page);
> > +     if (unlikely(!pgtable))
> > +             return NULL;
>
> AFAICS, above check only needs to be run once.
> It think we can move it to vmemmap_pgtable_free, can't we?

Yeah, we can. Thanks.

>
> > +     page_huge_pte(page) = list_first_entry_or_null(&pgtable->lru,
> > +                                                    struct page, lru);
> > +     if (page_huge_pte(page))
> > +             list_del(&pgtable->lru);
> > +     return page_to_virt(pgtable);
> > +}
>
> At the risk of adding more code, I think it would be nice to return a
> pagetable_t?
> So it is more coherent with the name of the function and with what
> we are doing.

Yeah.

>
> It is a pity we cannot converge these and pgtable_trans_huge_*.
> They share some code but it is different enough.
>
> > +static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> > +{
> > +     int i;
> > +     pte_t *pte_p;
> > +     unsigned int nr = nr_pgtable(h);
> > +
> > +     if (!nr)
> > +             return 0;
> > +
> > +     vmemmap_pgtable_init(page);
>
> Maybe just open code this one?

Sorry. I don't quite understand what it means. Could you explain?


>
> > +static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> > +{
> > +     pte_t *pte_p;
> > +
> > +     if (!nr_pgtable(h))
> > +             return;
> > +
> > +     while ((pte_p = vmemmap_pgtable_withdraw(page)))
> > +             pte_free_kernel(&init_mm, pte_p);
>
> As mentioned above, move the pgtable_t check from vmemmap_pgtable_withdraw
> in here.

OK.

>
>
> >  static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> >  {
> > +     /* 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);
> > @@ -1783,6 +1892,14 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
> >       if (!page)
> >               return NULL;
> >
> > +     if (vmemmap_pgtable_prealloc(h, page)) {
> > +             if (hstate_is_gigantic(h))
> > +                     free_gigantic_page(page, huge_page_order(h));
> > +             else
> > +                     put_page(page);
> > +             return NULL;
> > +     }
> > +
>
> I must confess I am bit puzzled.
>
> IIUC, in this patch we are just adding the helpers to create/tear the page
> tables.
> I do not think we actually need to call vmemmap_pgtable_prealloc/vmemmap_pgtable_free, do we?
> In the end, we are just allocating pages for pagetables and then free them shortly.
>
> I think it would make more sense to add the calls when they need to be?

OK,  will do. Thanks very much.

>
>
> --
> Oscar Salvador
> SUSE L3



--
Yours,
Muchun