.section .vectors, "ax", %progbits
__vectors_start:
    W(b)    vector_rst
    W(b)    vector_und
    W(ldr)    pc, __vectors_start + 0x1000
    W(b)    vector_pabt
    W(b)    vector_dabt--------------------------数据异常向量
    W(b)    vector_addrexcptn
    W(b)    vector_irq
    W(b)    vector_fiq

/*
 * Data abort dispatcher
 * Enter in ABT mode, spsr = USR CPSR, lr = USR PC
 */
    vector_stub    dabt, ABT_MODE, 8

    .long    __dabt_usr            @  0  (USR_26 / USR_32)
    .long    __dabt_invalid            @  1  (FIQ_26 / FIQ_32)
    .long    __dabt_invalid            @  2  (IRQ_26 / IRQ_32)
    .long    __dabt_svc            @  3  (SVC_26 / SVC_32)
...

__dabt_usr:
    usr_entry
    kuser_cmpxchg_check
    mov    r2, sp
    dabt_helper
    b    ret_from_exception
 UNWIND(.fnend        )
ENDPROC(__dabt_usr)


__dabt_svc:
    svc_entry
    mov    r2, sp
    dabt_helper
 THUMB(    ldr    r5, [sp, #S_PSR]    )    @ potentially updated CPSR
    svc_exit r5                @ return from exception
 UNWIND(.fnend        )
ENDPROC(__dabt_svc)



    .macro    dabt_helper

    @
    @ Call the processor-specific abort handler:
    @
    @  r2 - pt_regs
    @  r4 - aborted context pc
    @  r5 - aborted context psr
    @
    @ The abort handler must return the aborted address in r0, and
    @ the fault status register in r1.  r9 must be preserved.
    @
#ifdef MULTI_DABORT
    ldr    ip, .LCprocfns
    mov    lr, pc
    ldr    pc, [ip, #PROCESSOR_DABT_FUNC]
#else
    bl    CPU_DABORT_HANDLER--------------------------------指向v7_early_abort
#endif
    .endm

ENTRY(v7_early_abort)
    mrc    p15, 0, r1, c5, c0, 0        @ get FSR
    mrc    p15, 0, r0, c6, c0, 0        @ get FAR
...
    b    do_DataAbort
ENDPROC(v7_early_abort)

/*
 * Dispatch a data abort to the relevant handler.
 */
asmlinkage void __exception
do_DataAbort(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
{
    const struct fsr_info *inf = fsr_info + fsr_fs(fsr);----------------根据fsr从fsr_info中找到对应的处理函数。
    struct siginfo info;

    if (!inf->fn(addr, fsr & ~FSR_LNX_PF, regs))------------------------根据fsr进行处理
        return;

    pr_alert("Unhandled fault: %s (0x%03x) at 0x%08lx\n",---------------下面都是无法处理的异常
        inf->name, fsr, addr);
    show_pte(current->mm, addr);

    info.si_signo = inf->sig;
    info.si_errno = 0;
    info.si_code  = inf->code;
    info.si_addr  = (void __user *)addr;
    arm_notify_die("", regs, &info, fsr, 0);
}

static inline int fsr_fs(unsigned int fsr)
{
    return (fsr & FSR_FS3_0) | (fsr & FSR_FS4) >> 6;------------取fsr低4位；再与fsr第11位，然后右移6位，最后再与低4位或。
}

static struct fsr_info fsr_info[] = {
    /*
     * The following are the standard ARMv3 and ARMv4 aborts.  ARMv5
     * defines these to be "precise" aborts.
     */...
    { do_translation_fault,    SIGSEGV, SEGV_MAPERR,    "section translation fault"       },-----段转换错误，即找不到二级页表
    { do_bad,        SIGBUS,     0,        "external abort on linefetch"       },
    { do_page_fault,    SIGSEGV, SEGV_MAPERR,    "page translation fault"       },---------------页表错误，没有对应的物理地址
    { do_bad,        SIGBUS,     0,        "external abort on non-linefetch"  },
    { do_bad,        SIGSEGV, SEGV_ACCERR,    "section domain fault"           },
    { do_bad,        SIGBUS,     0,        "external abort on non-linefetch"  },
    { do_bad,        SIGSEGV, SEGV_ACCERR,    "page domain fault"           },
    { do_bad,        SIGBUS,     0,        "external abort on translation"       },
    { do_sect_fault,    SIGSEGV, SEGV_ACCERR,    "section permission fault"       },-------------段权限错误，二级页表权限错误
    { do_bad,        SIGBUS,     0,        "external abort on translation"       },
    { do_page_fault,    SIGSEGV, SEGV_ACCERR,    "page permission fault"           },------------页权限错误
...
}

static int __kprobes
do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
{
    struct task_struct *tsk;
    struct mm_struct *mm;
    int fault, sig, code;
    unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;

    if (notify_page_fault(regs, fsr))
        return 0;

    tsk = current;-------------------------------------------获取当前进程的task_struct
    mm  = tsk->mm;-------------------------------------------获取进程内存管理结构体mm_struct

    /* Enable interrupts if they were enabled in the parent context. */
    if (interrupts_enabled(regs))
        local_irq_enable();

    /*
     * If we‘re in an interrupt or have no user
     * context, we must not take the fault..
     */
    if (in_atomic() || !mm)----------------------------------in_atomic判断当前状态是否处于中断上下文或者禁止抢占，如果是跳转到no_context；如果当前进程没有mm，说明是一个内核线程，跳转到no_context。
        goto no_context;

    if (user_mode(regs))
        flags |= FAULT_FLAG_USER;
    if (fsr & FSR_WRITE)
        flags |= FAULT_FLAG_WRITE;

    /*
     * As per x86, we may deadlock here.  However, since the kernel only
     * validly references user space from well defined areas of the code,
     * we can bug out early if this is from code which shouldn‘t.
     */
    if (!down_read_trylock(&mm->mmap_sem)) {
        if (!user_mode(regs) && !search_exception_tables(regs->ARM_pc))----------发生在内核空间，且没有在exception tables查询到该地址，跳转到no_context。
            goto no_context;
retry:
        down_read(&mm->mmap_sem);---------------------------用户空间则睡眠等待锁持有者释放锁。
    } else {
        /*
         * The above down_read_trylock() might have succeeded in
         * which case, we‘ll have missed the might_sleep() from
         * down_read()
         */
        might_sleep();
#ifdef CONFIG_DEBUG_VM
        if (!user_mode(regs) &&
            !search_exception_tables(regs->ARM_pc))
            goto no_context;
#endif
    }

    fault = __do_page_fault(mm, addr, fsr, flags, tsk);

    /* If we need to retry but a fatal signal is pending, handle the
     * signal first. We do not need to release the mmap_sem because
     * it would already be released in __lock_page_or_retry in
     * mm/filemap.c. */
    if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
        return 0;

    /*
     * Major/minor page fault accounting is only done on the
     * initial attempt. If we go through a retry, it is extremely
     * likely that the page will be found in page cache at that point.
     */

    perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
    if (!(fault & VM_FAULT_ERROR) && flags & FAULT_FLAG_ALLOW_RETRY) {
        if (fault & VM_FAULT_MAJOR) {
            tsk->maj_flt++;
            perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
                    regs, addr);
        } else {
            tsk->min_flt++;
            perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
                    regs, addr);
        }
        if (fault & VM_FAULT_RETRY) {
            /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
            * of starvation. */
            flags &= ~FAULT_FLAG_ALLOW_RETRY;
            flags |= FAULT_FLAG_TRIED;
            goto retry;
        }
    }

    up_read(&mm->mmap_sem);

    /*
     * Handle the "normal" case first - VM_FAULT_MAJOR / VM_FAULT_MINOR
     */
    if (likely(!(fault & (VM_FAULT_ERROR | VM_FAULT_BADMAP | VM_FAULT_BADACCESS))))----没有错误，说明缺页中断处理完成。
        return 0;

    /*
     * If we are in kernel mode at this point, we
     * have no context to handle this fault with.
     */
    if (!user_mode(regs))-----------------------------------判断CPSR寄存器的低4位，CPSR的低5位表示当前所处的模式。如果低4位位0，则处于用户态。见下面CPSRM4~M0细节。
        goto no_context;------------------------------------进行内核空间错误处理

    if (fault & VM_FAULT_OOM) {
        /*
         * We ran out of memory, call the OOM killer, and return to
         * userspace (which will retry the fault, or kill us if we
         * got oom-killed)
         */
        pagefault_out_of_memory();--------------------------进行OOM处理，然后返回。
        return 0;
    }

    if (fault & VM_FAULT_SIGBUS) {
        /*
         * We had some memory, but were unable to
         * successfully fix up this page fault.
         */
        sig = SIGBUS;
        code = BUS_ADRERR;
    } else {
        /*
         * Something tried to access memory that
         * isn‘t in our memory map..
         */
        sig = SIGSEGV;
        code = fault == VM_FAULT_BADACCESS ?
            SEGV_ACCERR : SEGV_MAPERR;
    }

    __do_user_fault(tsk, addr, fsr, sig, code, regs);------用户模式下错误处理，通过给用户进程发信号：SIGBUS/SIGSEGV。
    return 0;

no_context:
    __do_kernel_fault(mm, addr, fsr, regs);----------------错误发生在内核模式，如果内核无法处理，此处产生oops错误。
    return 0;
}

/*
 * Different kinds of faults, as returned by handle_mm_fault().
 * Used to decide whether a process gets delivered SIGBUS or
 * just gets major/minor fault counters bumped up.
 */

#define VM_FAULT_MINOR    0 /* For backwards compat. Remove me quickly. */

#define VM_FAULT_OOM    0x0001
#define VM_FAULT_SIGBUS    0x0002
#define VM_FAULT_MAJOR    0x0004
#define VM_FAULT_WRITE    0x0008    /* Special case for get_user_pages */
#define VM_FAULT_HWPOISON 0x0010    /* Hit poisoned small page */
#define VM_FAULT_HWPOISON_LARGE 0x0020  /* Hit poisoned large page. Index encoded in upper bits */
#define VM_FAULT_SIGSEGV 0x0040

#define VM_FAULT_NOPAGE    0x0100    /* ->fault installed the pte, not return page */
#define VM_FAULT_LOCKED    0x0200    /* ->fault locked the returned page */
#define VM_FAULT_RETRY    0x0400    /* ->fault blocked, must retry */
#define VM_FAULT_FALLBACK 0x0800    /* huge page fault failed, fall back to small */

#define VM_FAULT_HWPOISON_LARGE_MASK 0xf000 /* encodes hpage index for large hwpoison */

#define VM_FAULT_ERROR    (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV |              VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE |              VM_FAULT_FALLBACK)

#define VM_FAULT_BADMAP 0x010000
#define VM_FAULT_BADACCESS    0x020000

static int __kprobes
__do_page_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
        unsigned int flags, struct task_struct *tsk)
{
    struct vm_area_struct *vma;
    int fault;

    vma = find_vma(mm, addr);--------------通过addr在查找vma，如果找不到则返回VM_FAULT_BADMAP错误。
    fault = VM_FAULT_BADMAP;
    if (unlikely(!vma))
        goto out;--------------------------返回VM_FAULT_BADMAP错误类型
    if (unlikely(vma->vm_start > addr))
        goto check_stack;

    /*
     * Ok, we have a good vm_area for this
     * memory access, so we can handle it.
     */
good_area:
    if (access_error(fsr, vma)) {---------判断当前vma是否可写或者可执行，如果否则返回VM_FAULT_BADACCESS错误。
        fault = VM_FAULT_BADACCESS;
        goto out;
    }

    return handle_mm_fault(mm, vma, addr & PAGE_MASK, flags);

check_stack:
    /* Don‘t allow expansion below FIRST_USER_ADDRESS */
    if (vma->vm_flags & VM_GROWSDOWN &&
        addr >= FIRST_USER_ADDRESS && !expand_stack(vma, addr))
        goto good_area;
out:
    return fault;
}


/*
 * By the time we get here, we already hold the mm semaphore
 *
 * The mmap_sem may have been released depending on flags and our
 * return value.  See filemap_fault() and __lock_page_or_retry().
 */
int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
            unsigned long address, unsigned int flags)
{
    int ret;

    __set_current_state(TASK_RUNNING);

    count_vm_event(PGFAULT);
    mem_cgroup_count_vm_event(mm, PGFAULT);

    /* do counter updates before entering really critical section. */
    check_sync_rss_stat(current);

    /*
     * Enable the memcg OOM handling for faults triggered in user
     * space.  Kernel faults are handled more gracefully.
     */
    if (flags & FAULT_FLAG_USER)
        mem_cgroup_oom_enable();

    ret = __handle_mm_fault(mm, vma, address, flags);

    if (flags & FAULT_FLAG_USER) {
        mem_cgroup_oom_disable();
                /*
                 * The task may have entered a memcg OOM situation but
                 * if the allocation error was handled gracefully (no
                 * VM_FAULT_OOM), there is no need to kill anything.
                 * Just clean up the OOM state peacefully.
                 */
                if (task_in_memcg_oom(current) && !(ret & VM_FAULT_OOM))
                        mem_cgroup_oom_synchronize(false);
    }

    return ret;
}

/*
 * By the time we get here, we already hold the mm semaphore
 *
 * The mmap_sem may have been released depending on flags and our
 * return value.  See filemap_fault() and __lock_page_or_retry().
 */
static int __handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
                 unsigned long address, unsigned int flags)
{
    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd;
    pte_t *pte;

    if (unlikely(is_vm_hugetlb_page(vma)))
        return hugetlb_fault(mm, vma, address, flags);

    pgd = pgd_offset(mm, address);------------------------------------获取当前address在当前进程页表项PGD页面目录项。
    pud = pud_alloc(mm, pgd, address);--------------------------------获取当前address在当前进程对应PUD页表目录项。
    if (!pud)
        return VM_FAULT_OOM;
    pmd = pmd_alloc(mm, pud, address);--------------------------------找到当前地址的PMD页表目录项
    if (!pmd)
        return VM_FAULT_OOM;
    if (pmd_none(*pmd) && transparent_hugepage_enabled(vma)) {
        int ret = VM_FAULT_FALLBACK;
        if (!vma->vm_ops)
            ret = do_huge_pmd_anonymous_page(mm, vma, address,
                    pmd, flags);
        if (!(ret & VM_FAULT_FALLBACK))
            return ret;
    } else {
        pmd_t orig_pmd = *pmd;
        int ret;

        barrier();
        if (pmd_trans_huge(orig_pmd)) {
            unsigned int dirty = flags & FAULT_FLAG_WRITE;

            /*
             * If the pmd is splitting, return and retry the
             * the fault.  Alternative: wait until the split
             * is done, and goto retry.
             */
            if (pmd_trans_splitting(orig_pmd))
                return 0;

            if (pmd_protnone(orig_pmd))
                return do_huge_pmd_numa_page(mm, vma, address,
                                 orig_pmd, pmd);

            if (dirty && !pmd_write(orig_pmd)) {
                ret = do_huge_pmd_wp_page(mm, vma, address, pmd,
                              orig_pmd);
                if (!(ret & VM_FAULT_FALLBACK))
                    return ret;
            } else {
                huge_pmd_set_accessed(mm, vma, address, pmd,
                              orig_pmd, dirty);
                return 0;
            }
        }
    }

    /*
     * Use __pte_alloc instead of pte_alloc_map, because we can‘t
     * run pte_offset_map on the pmd, if an huge pmd could
     * materialize from under us from a different thread.
     */
    if (unlikely(pmd_none(*pmd)) &&
        unlikely(__pte_alloc(mm, vma, pmd, address)))
        return VM_FAULT_OOM;
    /* if an huge pmd materialized from under us just retry later */
    if (unlikely(pmd_trans_huge(*pmd)))
        return 0;
    /*
     * A regular pmd is established and it can‘t morph into a huge pmd
     * from under us anymore at this point because we hold the mmap_sem
     * read mode and khugepaged takes it in write mode. So now it‘s
     * safe to run pte_offset_map().
     */
    pte = pte_offset_map(pmd, address);-------------------------------根据address从pmd中获取pte指针

    return handle_pte_fault(mm, vma, address, pte, pmd, flags);
}

/*
 * These routines also need to handle stuff like marking pages dirty
 * and/or accessed for architectures that don‘t do it in hardware (most
 * RISC architectures).  The early dirtying is also good on the i386.
 *
 * There is also a hook called "update_mmu_cache()" that architectures
 * with external mmu caches can use to update those (ie the Sparc or
 * PowerPC hashed page tables that act as extended TLBs).
 *
 * We enter with non-exclusive mmap_sem (to exclude vma changes,
 * but allow concurrent faults), and pte mapped but not yet locked.
 * We return with pte unmapped and unlocked.
 *
 * The mmap_sem may have been released depending on flags and our
 * return value.  See filemap_fault() and __lock_page_or_retry().
 */
static int handle_pte_fault(struct mm_struct *mm,
             struct vm_area_struct *vma, unsigned long address,
             pte_t *pte, pmd_t *pmd, unsigned int flags)
{
    pte_t entry;
    spinlock_t *ptl;

    /*
     * some architectures can have larger ptes than wordsize,
     * e.g.ppc44x-defconfig has CONFIG_PTE_64BIT=y and CONFIG_32BIT=y,
     * so READ_ONCE or ACCESS_ONCE cannot guarantee atomic accesses.
     * The code below just needs a consistent view for the ifs and
     * we later double check anyway with the ptl lock held. So here
     * a barrier will do.
     */
    entry = *pte;
    barrier();
    if (!pte_present(entry)) {------------------------------------------pte页表项中的L_PTE_PRESENT位没有置位，说明pte对应的物理页面不存在
        if (pte_none(entry)) {------------------------------------------pte页表项内容为空，同时pte对应物理页面也不存在
            if (vma->vm_ops) {
                if (likely(vma->vm_ops->fault))
                    return do_fault(mm, vma, address, pte,--------------vm_ops操作函数fault存在，则是文件映射页面异常中断
                            pmd, flags, entry);
            }
            return do_anonymous_page(mm, vma, address,------------------反之，vm_ops操作函数fault不存在，则是匿名页面异常中断
                         pte, pmd, flags);
        }
        return do_swap_page(mm, vma, address,---------------------------pte对应的物理页面不存在，但是pte页表项不为空，说明该页被交换到swap分区了
                    pte, pmd, flags, entry);
    }
======================================下面都是物理页面存在的情况===========================================
    if (pte_protnone(entry))
        return do_numa_page(mm, vma, address, entry, pte, pmd);

    ptl = pte_lockptr(mm, pmd);
    spin_lock(ptl);
    if (unlikely(!pte_same(*pte, entry)))
        goto unlock;
    if (flags & FAULT_FLAG_WRITE) {
        if (!pte_write(entry))------------------------------------------对只读属性的页面产生写异常，触发写时复制缺页中断
            return do_wp_page(mm, vma, address,
                    pte, pmd, ptl, entry);
        entry = pte_mkdirty(entry);
    }
    entry = pte_mkyoung(entry);
    if (ptep_set_access_flags(vma, address, pte, entry, flags & FAULT_FLAG_WRITE)) {
        update_mmu_cache(vma, address, pte);-----------------------------pte内容发生变化，需要把新的内容写入pte页表项中，并且刷新TLB和cache。
    } else {
        /*
         * This is needed only for protection faults but the arch code
         * is not yet telling us if this is a protection fault or not.
         * This still avoids useless tlb flushes for .text page faults
         * with threads.
         */
        if (flags & FAULT_FLAG_WRITE)
            flush_tlb_fix_spurious_fault(vma, address);
    }
unlock:
    pte_unmap_unlock(pte, ptl);
    return 0;
}

/*
 * We enter with non-exclusive mmap_sem (to exclude vma changes,
 * but allow concurrent faults), and pte mapped but not yet locked.
 * We return with mmap_sem still held, but pte unmapped and unlocked.
 */
static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
        unsigned long address, pte_t *page_table, pmd_t *pmd,
        unsigned int flags)
{
    struct mem_cgroup *memcg;
    struct page *page;
    spinlock_t *ptl;
    pte_t entry;

    pte_unmap(page_table);

    /* Check if we need to add a guard page to the stack */
    if (check_stack_guard_page(vma, address) < 0)
        return VM_FAULT_SIGSEGV;

    /* Use the zero-page for reads */
    if (!(flags & FAULT_FLAG_WRITE) && !mm_forbids_zeropage(mm)) {--------------如果是分配只读属性的页面，使用一个zeroed的全局页面empty_zero_page
        entry = pte_mkspecial(pfn_pte(my_zero_pfn(address),
                        vma->vm_page_prot));
        page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
        if (!pte_none(*page_table))
            goto unlock;
        goto setpte;------------------------------------------------------------跳转到setpte设置硬件pte表项，把新的PTE entry设置到硬件页表中
    }

    /* Allocate our own private page. */
    if (unlikely(anon_vma_prepare(vma)))
        goto oom;
    page = alloc_zeroed_user_highpage_movable(vma, address);-------------------如果页面是可写的，分配掩码是__GFP_MOVABLE|__GFP_WAIT|__GFP_IO|__GFP_FS|__GFP_HARDWALL|__GFP_HIGHMEM。最终调用alloc_pages，优先使用高端内存。
    if (!page)
        goto oom;
    /*
     * The memory barrier inside __SetPageUptodate makes sure that
     * preceeding stores to the page contents become visible before
     * the set_pte_at() write.
     */
    __SetPageUptodate(page);

    if (mem_cgroup_try_charge(page, mm, GFP_KERNEL, &memcg))
        goto oom_free_page;

    entry = mk_pte(page, vma->vm_page_prot);
    if (vma->vm_flags & VM_WRITE)
        entry = pte_mkwrite(pte_mkdirty(entry));-------------------------------生成一个新的PTE Entry

    page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
    if (!pte_none(*page_table))
        goto release;

    inc_mm_counter_fast(mm, MM_ANONPAGES);------------------------------------增加系统中匿名页面统计计数，计数类型是MM_ANONPAGES
    page_add_new_anon_rmap(page, vma, address);-------------------------------将匿名页面添加到RMAP系统中
    mem_cgroup_commit_charge(page, memcg, false);
    lru_cache_add_active_or_unevictable(page, vma);---------------------------将匿名页面添加到LRU链表中
setpte:
    set_pte_at(mm, address, page_table, entry);-------------------------------将entry设置到PTE硬件中

    /* No need to invalidate - it was non-present before */
    update_mmu_cache(vma, address, page_table);
unlock:
    pte_unmap_unlock(page_table, ptl);
    return 0;
release:
    mem_cgroup_cancel_charge(page, memcg);
    page_cache_release(page);
    goto unlock;
oom_free_page:
    page_cache_release(page);
oom:
    return VM_FAULT_OOM;
}

/*
 * We enter with non-exclusive mmap_sem (to exclude vma changes,
 * but allow concurrent faults).
 * The mmap_sem may have been released depending on flags and our
 * return value.  See filemap_fault() and __lock_page_or_retry().
 */
static int do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
        unsigned long address, pte_t *page_table, pmd_t *pmd,
        unsigned int flags, pte_t orig_pte)
{
    pgoff_t pgoff = (((address & PAGE_MASK)
            - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;

    pte_unmap(page_table);
    if (!(flags & FAULT_FLAG_WRITE))
        return do_read_fault(mm, vma, address, pmd, pgoff, flags,-------------------------只读异常
                orig_pte);
    if (!(vma->vm_flags & VM_SHARED))
        return do_cow_fault(mm, vma, address, pmd, pgoff, flags,--------------------------写时复制异常
                orig_pte);
    return do_shared_fault(mm, vma, address, pmd, pgoff, flags, orig_pte);----------------共享映射异常
}

static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma,
        unsigned long address, pmd_t *pmd,
        pgoff_t pgoff, unsigned int flags, pte_t orig_pte)
{
    struct page *fault_page;
    spinlock_t *ptl;
    pte_t *pte;
    int ret = 0;

    /*
     * Let‘s call ->map_pages() first and use ->fault() as fallback
     * if page by the offset is not ready to be mapped (cold cache or
     * something).
     */
    if (vma->vm_ops->map_pages && fault_around_bytes >> PAGE_SHIFT > 1) {------static unsigned long fault_around_bytes __read_mostly =rounddown_pow_of_two(65536);

        pte = pte_offset_map_lock(mm, pmd, address, &ptl);        do_fault_around(vma, address, pte, pgoff, flags);----------------------围绕在缺页异常地址周围提前映射尽可能多的页面，提前建立进程地址空间和page cache的映射关系有利于减少发生缺页终端的次数。这里只是和现存的page cache提前建立映射关系，而不会去创建page cache。
        if (!pte_same(*pte, orig_pte))
            goto unlock_out;
        pte_unmap_unlock(pte, ptl);
    }

    ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);-----------创建page cache的页面实际操作
    if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
        return ret;

    pte = pte_offset_map_lock(mm, pmd, address, &ptl);
    if (unlikely(!pte_same(*pte, orig_pte))) {
        pte_unmap_unlock(pte, ptl);
        unlock_page(fault_page);
        page_cache_release(fault_page);
        return ret;
    }
    do_set_pte(vma, address, fault_page, pte, false, false);-------------------生成新的PTE Entry设置到硬件页表项中
    unlock_page(fault_page);
unlock_out:
    pte_unmap_unlock(pte, ptl);
    return ret;
}

static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
        unsigned long address, pmd_t *pmd,
        pgoff_t pgoff, unsigned int flags, pte_t orig_pte)
{
    struct page *fault_page, *new_page;
    struct mem_cgroup *memcg;
    spinlock_t *ptl;
    pte_t *pte;
    int ret;

    if (unlikely(anon_vma_prepare(vma)))
        return VM_FAULT_OOM;

    new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);----------------优先从高端内存分配可移动页面
    if (!new_page)
        return VM_FAULT_OOM;

    if (mem_cgroup_try_charge(new_page, mm, GFP_KERNEL, &memcg)) {
        page_cache_release(new_page);
        return VM_FAULT_OOM;
    }

    ret = __do_fault(vma, address, pgoff, flags, new_page, &fault_page);----------利用vma->vm_ops->fault()读取文件内容到fault_page中。
    if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
        goto uncharge_out;

    if (fault_page)
        copy_user_highpage(new_page, fault_page, address, vma);-------------------将fault_page页面内容复制到新分配页面new_page中。
    __SetPageUptodate(new_page);

    pte = pte_offset_map_lock(mm, pmd, address, &ptl);
    if (unlikely(!pte_same(*pte, orig_pte))) {------------------------------------如果pte和orig_pte不一致，说明中间有人修改了pte，那么释放fault_page和new_page页面并退出。
        pte_unmap_unlock(pte, ptl);
        if (fault_page) {
            unlock_page(fault_page);
            page_cache_release(fault_page);
        } else {
            /*
             * The fault handler has no page to lock, so it holds
             * i_mmap_lock for read to protect against truncate.
             */
            i_mmap_unlock_read(vma->vm_file->f_mapping);
        }
        goto uncharge_out;
    }
    do_set_pte(vma, address, new_page, pte, true, true);-------------------------将PTE Entry设置到PTE硬件页表项pte中。
    mem_cgroup_commit_charge(new_page, memcg, false);
    lru_cache_add_active_or_unevictable(new_page, vma);--------------------------将新分配的new_page加入到LRU链表中。
    pte_unmap_unlock(pte, ptl);
    if (fault_page) {
        unlock_page(fault_page);
        page_cache_release(fault_page);-------------------------------------------释放fault_page页面
    } else {
        /*
         * The fault handler has no page to lock, so it holds
         * i_mmap_lock for read to protect against truncate.
         */
        i_mmap_unlock_read(vma->vm_file->f_mapping);
    }
    return ret;
uncharge_out:
    mem_cgroup_cancel_charge(new_page, memcg);
    page_cache_release(new_page);
    return ret;
}

static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
        unsigned long address, pmd_t *pmd,
        pgoff_t pgoff, unsigned int flags, pte_t orig_pte)
{
    struct page *fault_page;
    struct address_space *mapping;
    spinlock_t *ptl;
    pte_t *pte;
    int dirtied = 0;
    int ret, tmp;

    ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);-----------------读取文件到fault_page中
    if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
        return ret;

    /*
     * Check if the backing address space wants to know that the page is
     * about to become writable
     */
    if (vma->vm_ops->page_mkwrite) {
        unlock_page(fault_page);
        tmp = do_page_mkwrite(vma, fault_page, address);-----------------------------通知进程地址空间，fault_page将变成可写的，那么进程可能需要等待这个page的内容回写成功。
        if (unlikely(!tmp ||
                (tmp & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
            page_cache_release(fault_page);
            return tmp;
        }
    }

    pte = pte_offset_map_lock(mm, pmd, address, &ptl);
    if (unlikely(!pte_same(*pte, orig_pte))) {---------------------------------------判断该异常地址对应的硬件页表项pte内容与之前的orig_pte是否一致。不一致，就需要释放fault_page。
        pte_unmap_unlock(pte, ptl);
        unlock_page(fault_page);
        page_cache_release(fault_page);
        return ret;
    }
    do_set_pte(vma, address, fault_page, pte, true, false);--------------------------利用fault_page新生成一个PTE Entry并设置到页表项pte中
    pte_unmap_unlock(pte, ptl);

    if (set_page_dirty(fault_page))--------------------------------------------------设置页面为脏
        dirtied = 1;
    /*
     * Take a local copy of the address_space - page.mapping may be zeroed
     * by truncate after unlock_page().   The address_space itself remains
     * pinned by vma->vm_file‘s reference.  We rely on unlock_page()‘s
     * release semantics to prevent the compiler from undoing this copying.
     */
    mapping = fault_page->mapping;
    unlock_page(fault_page);
    if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {
        /*
         * Some device drivers do not set page.mapping but still
         * dirty their pages
         */
        balance_dirty_pages_ratelimited(mapping);------------------------------------每设置一页为dirty，检查是否需要回写；如需要则回写一部分页面
    }

    if (!vma->vm_ops->page_mkwrite)
        file_update_time(vma->vm_file);

    return ret;
}

/*
 * This routine handles present pages, when users try to write
 * to a shared page. It is done by copying the page to a new address
 * and decrementing the shared-page counter for the old page.
 */
static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
        unsigned long address, pte_t *page_table, pmd_t *pmd,
        spinlock_t *ptl, pte_t orig_pte)
    __releases(ptl)
{
    struct page *old_page, *new_page = NULL;
    pte_t entry;
    int ret = 0;
    int page_mkwrite = 0;
    bool dirty_shared = false;
    unsigned long mmun_start = 0;    /* For mmu_notifiers */
    unsigned long mmun_end = 0;    /* For mmu_notifiers */
    struct mem_cgroup *memcg;

    old_page = vm_normal_page(vma, address, orig_pte);--------------------------查找缺页异常地址address对应页面的struct page数据结构，返回normal mapping页面。
    if (!old_page) {------------------------------------------------------------如果返回old_page为NULL，说明这时一个special mapping页面。
        /*
         * VM_MIXEDMAP !pfn_valid() case, or VM_SOFTDIRTY clear on a
         * VM_PFNMAP VMA.
         *
         * We should not cow pages in a shared writeable mapping.
         * Just mark the pages writable as we can‘t do any dirty
         * accounting on raw pfn maps.
         */
        if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
                     (VM_WRITE|VM_SHARED))
            goto reuse;
        goto gotten;
    }

    /*
     * Take out anonymous pages first, anonymous shared vmas are
     * not dirty accountable.
     */
    if (PageAnon(old_page) && !PageKsm(old_page)) {-----------------------------针对匿名非KSM页面之外的情况进行进行处理，
        if (!trylock_page(old_page)) {
            page_cache_get(old_page);
            pte_unmap_unlock(page_table, ptl);
            lock_page(old_page);
            page_table = pte_offset_map_lock(mm, pmd, address,
                             &ptl);
            if (!pte_same(*page_table, orig_pte)) {
                unlock_page(old_page);
                goto unlock;
            }
            page_cache_release(old_page);
        }
        if (reuse_swap_page(old_page)) {
            /*
             * The page is all ours.  Move it to our anon_vma so
             * the rmap code will not search our parent or siblings.
             * Protected against the rmap code by the page lock.
             */
            page_move_anon_rmap(old_page, vma, address);
            unlock_page(old_page);
            goto reuse;
        }
        unlock_page(old_page);
    } else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==---------------处理匿名非KSM页面之外的情况
                    (VM_WRITE|VM_SHARED))) {
        page_cache_get(old_page);
        /*
         * Only catch write-faults on shared writable pages,
         * read-only shared pages can get COWed by
         * get_user_pages(.write=1, .force=1).
         */
        if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
            int tmp;

            pte_unmap_unlock(page_table, ptl);
            tmp = do_page_mkwrite(vma, old_page, address);
            if (unlikely(!tmp || (tmp &
                    (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
                page_cache_release(old_page);
                return tmp;
            }
            /*
             * Since we dropped the lock we need to revalidate
             * the PTE as someone else may have changed it.  If
             * they did, we just return, as we can count on the
             * MMU to tell us if they didn‘t also make it writable.
             */
            page_table = pte_offset_map_lock(mm, pmd, address,
                             &ptl);
            if (!pte_same(*page_table, orig_pte)) {
                unlock_page(old_page);
                goto unlock;
            }
            page_mkwrite = 1;
        }

        dirty_shared = true;

reuse:
        /*
         * Clear the pages cpupid information as the existing
         * information potentially belongs to a now completely
         * unrelated process.
         */
        if (old_page)
            page_cpupid_xchg_last(old_page, (1 << LAST_CPUPID_SHIFT) - 1);

        flush_cache_page(vma, address, pte_pfn(orig_pte));
        entry = pte_mkyoung(orig_pte);
        entry = maybe_mkwrite(pte_mkdirty(entry), vma);
        if (ptep_set_access_flags(vma, address, page_table, entry,1))
            update_mmu_cache(vma, address, page_table);
        pte_unmap_unlock(page_table, ptl);
        ret |= VM_FAULT_WRITE;

        if (dirty_shared) {
            struct address_space *mapping;
            int dirtied;

            if (!page_mkwrite)
                lock_page(old_page);

            dirtied = set_page_dirty(old_page);
            VM_BUG_ON_PAGE(PageAnon(old_page), old_page);
            mapping = old_page->mapping;
            unlock_page(old_page);
            page_cache_release(old_page);

            if ((dirtied || page_mkwrite) && mapping) {
                /*
                 * Some device drivers do not set page.mapping
                 * but still dirty their pages
                 */
                balance_dirty_pages_ratelimited(mapping);
            }

            if (!page_mkwrite)
                file_update_time(vma->vm_file);
        }

        return ret;
    }

    /*
     * Ok, we need to copy. Oh, well..
     */
    page_cache_get(old_page);
gotten:------------------------------------------------------------------------表示需要新建一个页面，也就是写时复制。
    pte_unmap_unlock(page_table, ptl);

    if (unlikely(anon_vma_prepare(vma)))
        goto oom;

    if (is_zero_pfn(pte_pfn(orig_pte))) {
        new_page = alloc_zeroed_user_highpage_movable(vma, address);----------分配高端、可移动、零页面
        if (!new_page)
            goto oom;
    } else {
        new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);--------分配高端、可移动页面
        if (!new_page)
            goto oom;
        cow_user_page(new_page, old_page, address, vma);
    }
    __SetPageUptodate(new_page);

    if (mem_cgroup_try_charge(new_page, mm, GFP_KERNEL, &memcg))
        goto oom_free_new;

    mmun_start  = address & PAGE_MASK;
    mmun_end    = mmun_start + PAGE_SIZE;
    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

    /*
     * Re-check the pte - we dropped the lock
     */
    page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
    if (likely(pte_same(*page_table, orig_pte))) {
        if (old_page) {
            if (!PageAnon(old_page)) {
                dec_mm_counter_fast(mm, MM_FILEPAGES);
                inc_mm_counter_fast(mm, MM_ANONPAGES);
            }
        } else
            inc_mm_counter_fast(mm, MM_ANONPAGES);
        flush_cache_page(vma, address, pte_pfn(orig_pte));
        entry = mk_pte(new_page, vma->vm_page_prot);---------------------------利用new_page和vma生成PTE Entry
        entry = maybe_mkwrite(pte_mkdirty(entry), vma);
        /*
         * Clear the pte entry and flush it first, before updating the
         * pte with the new entry. This will avoid a race condition
         * seen in the presence of one thread doing SMC and another
         * thread doing COW.
         */
        ptep_clear_flush_notify(vma, address, page_table);
        page_add_new_anon_rmap(new_page, vma, address);------------------------把new_page添加到RMAP反向映射机制，设置页面计数_mapcount为0。
        mem_cgroup_commit_charge(new_page, memcg, false);
        lru_cache_add_active_or_unevictable(new_page, vma);--------------------把new_page添加到活跃的LRU链表中
        /*
         * We call the notify macro here because, when using secondary
         * mmu page tables (such as kvm shadow page tables), we want the
         * new page to be mapped directly into the secondary page table.
         */
        set_pte_at_notify(mm, address, page_table, entry);
        update_mmu_cache(vma, address, page_table);
        if (old_page) {
            /*
             * Only after switching the pte to the new page may
             * we remove the mapcount here. Otherwise another
             * process may come and find the rmap count decremented
             * before the pte is switched to the new page, and
             * "reuse" the old page writing into it while our pte
             * here still points into it and can be read by other
             * threads.
             *
             * The critical issue is to order this
             * page_remove_rmap with the ptp_clear_flush above.
             * Those stores are ordered by (if nothing else,)
             * the barrier present in the atomic_add_negative
             * in page_remove_rmap.
             *
             * Then the TLB flush in ptep_clear_flush ensures that
             * no process can access the old page before the
             * decremented mapcount is visible. And the old page
             * cannot be reused until after the decremented
             * mapcount is visible. So transitively, TLBs to
             * old page will be flushed before it can be reused.
             */
            page_remove_rmap(old_page);--------------------------------------_mapcount计数减1
        }

        /* Free the old page.. */
        new_page = old_page;
        ret |= VM_FAULT_WRITE;
    } else
        mem_cgroup_cancel_charge(new_page, memcg);

    if (new_page)
        page_cache_release(new_page);----------------------------------------释放new_page，这里new_page==old_page。
unlock:
    pte_unmap_unlock(page_table, ptl);
    if (mmun_end > mmun_start)
        mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
    if (old_page) {
        /*
         * Don‘t let another task, with possibly unlocked vma,
         * keep the mlocked page.
         */
        if ((ret & VM_FAULT_WRITE) && (vma->vm_flags & VM_LOCKED)) {
            lock_page(old_page);    /* LRU manipulation */
            munlock_vma_page(old_page);
            unlock_page(old_page);
        }
        page_cache_release(old_page);
    }
    return ret;
oom_free_new:
    page_cache_release(new_page);
oom:
    if (old_page)
        page_cache_release(old_page);
    return VM_FAULT_OOM;
}