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SubjectRe: [RFC PATCH 21/24] mm/hugetlb: Merge pte to huge pmd only for gigantic page
On Tue, Sep 15, 2020 at 9:03 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> Merge pte to huge pmd if it has ever been split. Now only support
> gigantic page which's vmemmap pages size is an integer multiple of
> PMD_SIZE. This is the simplest case to handle.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
> include/linux/hugetlb.h | 7 +++
> mm/hugetlb.c | 104 +++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 109 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index e3aa192f1c39..c56df0da7ae5 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -611,6 +611,13 @@ static inline bool vmemmap_pmd_huge(pmd_t *pmd)
> }
> #endif
>
> +#ifndef vmemmap_pmd_mkhuge
> +static inline pmd_t vmemmap_pmd_mkhuge(struct page *page)
> +{
> + return pmd_mkhuge(mk_pmd(page, PAGE_KERNEL));
> +}
> +#endif
> +
> #ifndef VMEMMAP_HPAGE_SHIFT
> #define VMEMMAP_HPAGE_SHIFT PMD_SHIFT
> #endif
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 28c154679838..3ca36e259b4e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1759,6 +1759,62 @@ static void __remap_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
> }
> }
>
> +static void __replace_huge_page_pte_vmemmap(pte_t *ptep, unsigned long start,
> + unsigned int nr, struct page *huge,
> + struct list_head *free_pages)
> +{
> + unsigned long addr;
> + unsigned long end = start + (nr << PAGE_SHIFT);
> +
> + for (addr = start; addr < end; addr += PAGE_SIZE, ptep++) {
> + struct page *page;
> + pte_t old = *ptep;
> + pte_t entry;
> +
> + prepare_vmemmap_page(huge);
> +
> + entry = mk_pte(huge++, PAGE_KERNEL);
> + VM_WARN_ON(!pte_present(old));
> + page = pte_page(old);
> + list_add(&page->lru, free_pages);
> +
> + set_pte_at(&init_mm, addr, ptep, entry);
> + }
> +}
> +
> +static void replace_huge_page_pmd_vmemmap(pmd_t *pmd, unsigned long start,
> + struct page *huge,
> + struct list_head *free_pages)
> +{
> + unsigned long end = start + VMEMMAP_HPAGE_SIZE;
> +
> + flush_cache_vunmap(start, end);
> + __replace_huge_page_pte_vmemmap(pte_offset_kernel(pmd, start), start,
> + VMEMMAP_HPAGE_NR, huge, free_pages);
> + flush_tlb_kernel_range(start, end);
> +}
> +
> +static pte_t *merge_vmemmap_pte(pmd_t *pmdp, unsigned long addr)
> +{
> + pte_t *pte;
> + struct page *page;
> +
> + pte = pte_offset_kernel(pmdp, addr);
> + page = pte_page(*pte);
> + set_pmd(pmdp, vmemmap_pmd_mkhuge(page));
> +
> + return pte;
> +}
> +
> +static void merge_huge_page_pmd_vmemmap(pmd_t *pmd, unsigned long start,
> + struct page *huge,
> + struct list_head *free_pages)
> +{
> + replace_huge_page_pmd_vmemmap(pmd, start, huge, free_pages);
> + pte_free_kernel(&init_mm, merge_vmemmap_pte(pmd, start));
> + flush_tlb_kernel_range(start, start + VMEMMAP_HPAGE_SIZE);
> +}
> +
> static inline void alloc_vmemmap_pages(struct hstate *h, struct list_head *list)
> {
> int i;
> @@ -1772,6 +1828,15 @@ static inline void alloc_vmemmap_pages(struct hstate *h, struct list_head *list)
> }
> }
>
> +static inline void dissolve_compound_page(struct page *page, unsigned int order)
> +{
> + int i;
> + unsigned int nr_pages = 1 << order;
> +
> + for (i = 1; i < nr_pages; i++)
> + set_page_refcounted(page + i);
> +}
> +
> static void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> {
> pmd_t *pmd;
> @@ -1791,10 +1856,45 @@ static void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> __remap_huge_page_pte_vmemmap);
> if (!freed_vmemmap_hpage_dec(pmd_page(*pmd)) && pmd_split(pmd)) {
> /*
> - * Todo:
> - * Merge pte to huge pmd if it has ever been split.
> + * Merge pte to huge pmd if it has ever been split. Now only
> + * support gigantic page which's vmemmap pages size is an
> + * integer multiple of PMD_SIZE. This is the simplest case
> + * to handle.
> */
> clear_pmd_split(pmd);
> +
> + if (IS_ALIGNED(nr_vmemmap(h), VMEMMAP_HPAGE_NR)) {
> + unsigned long addr = (unsigned long)head;
> + unsigned long end = addr + nr_vmemmap_size(h);
> +
> + spin_unlock(ptl);
> +
> + for (; addr < end; addr += VMEMMAP_HPAGE_SIZE) {
> + void *to;
> + struct page *page;
> +
> + page = alloc_pages(GFP_VMEMMAP_PAGE & ~__GFP_NOFAIL,
> + VMEMMAP_HPAGE_ORDER);
> + if (!page)
> + goto out;

Here forget to call dissolve_compound_page().

+ dissolve_compound_page(page,
+ VMEMMAP_HPAGE_ORDER);

> +
> + to = page_to_virt(page);
> + memcpy(to, (void *)addr, VMEMMAP_HPAGE_SIZE);
> +
> + /*
> + * Make sure that any data that writes to the
> + * @to is made visible to the physical page.
> + */
> + flush_kernel_vmap_range(to, VMEMMAP_HPAGE_SIZE);
> +
> + merge_huge_page_pmd_vmemmap(pmd++, addr, page,
> + &remap_pages);
> + }
> +
> +out:
> + free_vmemmap_page_list(&remap_pages);
> + return;
> + }
> }
> spin_unlock(ptl);
> }
> --
> 2.20.1
>


--
Yours,
Muchun

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