Re: [External] Re: [PATCH v2 05/19] mm/hugetlb: Introduce pgtable allocation/freeing helpers
From: Muchun Song
Date: Wed Oct 28 2020 - 18:25:10 EST
On Wed, Oct 28, 2020 at 8:33 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 10/26/20 7:51 AM, Muchun Song wrote:
> > On some architectures, the vmemmap areas use huge page mapping.
> > 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
> > huge pmd.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> > arch/x86/include/asm/hugetlb.h | 5 ++
> > include/linux/hugetlb.h | 17 +++++
> > mm/hugetlb.c | 117 +++++++++++++++++++++++++++++++++
> > 3 files changed, 139 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/hugetlb.h b/arch/x86/include/asm/hugetlb.h
> > index 1721b1aadeb1..f5e882f999cd 100644
> > --- a/arch/x86/include/asm/hugetlb.h
> > +++ b/arch/x86/include/asm/hugetlb.h
> > @@ -5,6 +5,11 @@
> > #include <asm/page.h>
> > #include <asm-generic/hugetlb.h>
> >
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#define VMEMMAP_HPAGE_SHIFT PMD_SHIFT
> > +#define arch_vmemmap_support_huge_mapping() boot_cpu_has(X86_FEATURE_PSE)
> > +#endif
> > +
> > #define hugepages_supported() boot_cpu_has(X86_FEATURE_PSE)
> >
> > #endif /* _ASM_X86_HUGETLB_H */
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index eed3dd3bd626..ace304a6196c 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -593,6 +593,23 @@ static inline unsigned int blocks_per_huge_page(struct hstate *h)
> >
> > #include <asm/hugetlb.h>
> >
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#ifndef arch_vmemmap_support_huge_mapping
> > +static inline bool arch_vmemmap_support_huge_mapping(void)
> > +{
> > + return false;
> > +}
> > +#endif
> > +
> > +#ifndef VMEMMAP_HPAGE_SHIFT
> > +#define VMEMMAP_HPAGE_SHIFT PMD_SHIFT
> > +#endif
> > +#define VMEMMAP_HPAGE_ORDER (VMEMMAP_HPAGE_SHIFT - PAGE_SHIFT)
> > +#define VMEMMAP_HPAGE_NR (1 << VMEMMAP_HPAGE_ORDER)
> > +#define VMEMMAP_HPAGE_SIZE ((1UL) << VMEMMAP_HPAGE_SHIFT)
> > +#define VMEMMAP_HPAGE_MASK (~(VMEMMAP_HPAGE_SIZE - 1))
> > +#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> > +
> > #ifndef is_hugepage_only_range
> > static inline int is_hugepage_only_range(struct mm_struct *mm,
> > unsigned long addr, unsigned long len)
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index f1b2b733b49b..d6ae9b6876be 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1295,11 +1295,108 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> > #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > #define RESERVE_VMEMMAP_NR 2U
> >
> > +#define page_huge_pte(page) ((page)->pmd_huge_pte)
> > +
>
> I am not good at function names. The following suggestions may be too
> verbose. However, they helped me understand purpose of routines.
>
> > static inline unsigned int nr_free_vmemmap(struct hstate *h)
>
> perhaps? free_vmemmap_pages_per_hpage()
>
> > {
> > return h->nr_free_vmemmap_pages;
> > }
> >
> > +static inline unsigned int nr_vmemmap(struct hstate *h)
>
> perhaps? vmemmap_pages_per_hpage()
>
> > +{
> > + return nr_free_vmemmap(h) + RESERVE_VMEMMAP_NR;
> > +}
> > +
> > +static inline unsigned long nr_vmemmap_size(struct hstate *h)
>
> perhaps? vmemmap_pages_size_per_hpage()
>
> > +{
> > + return (unsigned long)nr_vmemmap(h) << PAGE_SHIFT;
> > +}
> > +
> > +static inline unsigned int nr_pgtable(struct hstate *h)
>
> perhaps? pgtable_pages_to_prealloc_per_hpage()
Good suggestions. Thanks. I will apply this.
>
> > +{
> > + 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;
> > +}
> > +
> > +static inline void vmemmap_pgtable_init(struct page *page)
> > +{
> > + page_huge_pte(page) = NULL;
> > +}
> > +
>
> I see the following routines follow the pattern for vmemmap manipulation
> in dax.
Did you mean move those functions to mm/sparse-vmemmap.c?
>
> > +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;
> > +}
> > +
> > +static pte_t *vmemmap_pgtable_withdraw(struct page *page)
> > +{
> > + pgtable_t pgtable;
> > +
> > + /* FIFO */
> > + pgtable = page_huge_pte(page);
> > + if (unlikely(!pgtable))
> > + return NULL;
> > + 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);
> > +}
> > +
> > +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);
> > +
> > + for (i = 0; i < nr; i++) {
> > + pte_p = pte_alloc_one_kernel(&init_mm);
> > + if (!pte_p)
> > + goto out;
> > + vmemmap_pgtable_deposit(page, pte_p);
> > + }
> > +
> > + return 0;
> > +out:
> > + while (i-- && (pte_p = vmemmap_pgtable_withdraw(page)))
> > + pte_free_kernel(&init_mm, pte_p);
> > + return -ENOMEM;
> > +}
> > +
> > +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);
> > +}
> > +
> > static void __init hugetlb_vmemmap_init(struct hstate *h)
> > {
> > unsigned int order = huge_page_order(h);
> > @@ -1323,6 +1420,15 @@ static void __init hugetlb_vmemmap_init(struct hstate *h)
> > static inline void hugetlb_vmemmap_init(struct hstate *h)
> > {
> > }
> > +
> > +static inline int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> > +{
> > + return 0;
> > +}
> > +
> > +static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> > +{
> > +}
> > #endif
> >
> > static void update_and_free_page(struct hstate *h, struct page *page)
> > @@ -1531,6 +1637,9 @@ void free_huge_page(struct page *page)
> >
> > 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;
> > + }
> > +
>
> It seems a bit strange that we will fail a huge page allocation if
> vmemmap_pgtable_prealloc fails. Not sure, but it almost seems like we shold
> allow the allocation and log a warning? It is somewhat unfortunate that
> we need to allocate a page to free pages.
Yeah, it seems unfortunate. But if we allocate success, we can free some
vmemmap pages later. Like a compromise :) . If we can successfully allocate
a huge page, I also prefer to be able to successfully allocate another one page.
If we allow the allocation when vmemmap_pgtable_prealloc fails, we also
need to mark this page that vmemmap has not been released. Seems
increase complexity.
Thanks.
>
> > if (hstate_is_gigantic(h))
> > prep_compound_gigantic_page(page, huge_page_order(h));
> > prep_new_huge_page(h, page, page_to_nid(page));
> >
>
>
> --
> Mike Kravetz
--
Yours,
Muchun