Yrurtj

I've been trying to understand how fork() works and finally arrived at copy_pte_range(). Most of the functions are understandable but few are quite questionable.

Kernel: 4.14.84

static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,

           pmd_t *dst_pmd, pmd_t *src_pmd, struct vm_area_struct *vma,

           unsigned long addr, unsigned long end)

{

    pte_t *orig_src_pte, *orig_dst_pte;

    pte_t *src_pte, *dst_pte;

    spinlock_t *src_ptl, *dst_ptl;

    int progress = 0;

    int rss[NR_MM_COUNTERS];

    swp_entry_t entry = (swp_entry_t){0};



again:

    init_rss_vec(rss);



    dst_pte = pte_alloc_map_lock(dst_mm, dst_pmd, addr, &dst_ptl);

    if (!dst_pte)

        return -ENOMEM;

    src_pte = pte_offset_map(src_pmd, addr);

    src_ptl = pte_lockptr(src_mm, src_pmd);

    spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);

    orig_src_pte = src_pte;

    orig_dst_pte = dst_pte;

    arch_enter_lazy_mmu_mode();



    do {

        /*

         * We are holding two locks at this point - either of them

         * could generate latencies in another task on another CPU.

         */

        if (progress >= 32) {

            progress = 0;

            if (need_resched() ||

                spin_needbreak(src_ptl) || spin_needbreak(dst_ptl))

                break;

        }

        if (pte_none(*src_pte)) {

            progress++;

            continue;

        }

        entry.val = copy_one_pte(dst_mm, src_mm, dst_pte, src_pte,

                            vma, addr, rss);

        if (entry.val)

            break;

        progress += 8;

    } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);



    arch_leave_lazy_mmu_mode();

    spin_unlock(src_ptl);

    pte_unmap(orig_src_pte);

    add_mm_rss_vec(dst_mm, rss);

    pte_unmap_unlock(orig_dst_pte, dst_ptl);

    cond_resched();



    if (entry.val) {

        if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)

            return -ENOMEM;

        progress = 0;

    }

    if (addr != end)

        goto again;

    return 0;

}

Questions

1. In the do {} while(), what is the purpose of progress variable?

2. After do {} while(), there is pte_unmap(orig_src_pte); Why is it needed? This is the process of fork(). Based on my knowledge, the parent pte(orig_src_pte) should still be mapped because the process is based on Copy-on-Write so I guess it doesn't have to be unmapped.