Re: [PATCH v8 04/12] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

Oscar Salvador
2020-12-10T14:57:29Z

On Thu, Dec 10, 2020 at 11:55:18AM +0800, Muchun Song wrote:
> The free_vmemmap_pages_per_hpage() which indicate that how many vmemmap
> pages associated with a HugeTLB page that can be freed to the buddy
> allocator just returns zero now, because all infrastructure is not
> ready. Once all the infrastructure is ready, we will rework this
> function to support the feature.

I would reword the above to:

"free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled.
We will enable it once all the infrastructure is there."

Or something along those lines.

> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Overall this looks good to me, and it has seen a considerable
simplification, which is good.
Some nits/questions below:

> +#define vmemmap_hpage_addr_end(addr, end) \
> +({ \
> + unsigned long __boundary; \
> + __boundary = ((addr) + VMEMMAP_HPAGE_SIZE) & VMEMMAP_HPAGE_MASK; \
> + (__boundary - 1 < (end) - 1) ? __boundary : (end); \
> +})

Maybe add a little comment explaining what are you trying to get here.

> +/*
> + * Walk a vmemmap address to the pmd it maps.
> + */
> +static pmd_t *vmemmap_to_pmd(unsigned long addr)
> +{
> + pgd_t *pgd;
> + p4d_t *p4d;
> + pud_t *pud;
> + pmd_t *pmd;
> +
> + pgd = pgd_offset_k(addr);
> + if (pgd_none(*pgd))
> + return NULL;
> +
> + p4d = p4d_offset(pgd, addr);
> + if (p4d_none(*p4d))
> + return NULL;
> +
> + pud = pud_offset(p4d, addr);
> + if (pud_none(*pud))
> + return NULL;
> +
> + pmd = pmd_offset(pud, addr);
> + if (pmd_none(*pmd))
> + return NULL;
> +
> + return pmd;
> +}

I saw that some people suggested to put all the non-hugetlb vmemmap
functions under sparsemem-vmemmap.c, which makes some sense if some
feature is going to re-use this code somehow. (I am not sure if the
recent patches that take advantage of this feature for ZONE_DEVICE needs
something like this).

I do not have a strong opinion on this though.

> +static void vmemmap_reuse_pte_range(struct page *reuse, pte_t *pte,
> + unsigned long start, unsigned long end,
> + struct list_head *vmemmap_pages)
> +{
> + /*
> + * Make the tail pages are mapped with read-only to catch
> + * illegal write operation to the tail pages.
> + */
> + pgprot_t pgprot = PAGE_KERNEL_RO;
> + pte_t entry = mk_pte(reuse, pgprot);
> + unsigned long addr;
> +
> + for (addr = start; addr < end; addr += PAGE_SIZE, pte++) {
> + struct page *page;
> +
> + VM_BUG_ON(pte_none(*pte));

If it is none, page will be NULL and we will crash in the list_add
below?

> +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> + struct list_head *vmemmap_pages)
> +{
> + pmd_t *pmd;
> + unsigned long next, addr = start;
> + struct page *reuse = NULL;
> +
> + VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> + VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> + VM_BUG_ON((start >> PUD_SHIFT) != (end >> PUD_SHIFT));
This last VM_BUG_ON, is to see if both fall under the same PUD table?

> +
> + pmd = vmemmap_to_pmd(addr);
> + BUG_ON(!pmd);

Which is the criteria you followed to make this BUG_ON and VM_BUG_ON
in the check from vmemmap_reuse_pte_range?

--
Oscar Salvador
SUSE L3