current-> mm在Linux内核中为NULL [英] current->mm gives NULL in linux kernel

问题描述

我想遍历页面表，所以我已经访问了current-> mm，但是它给出了NULL值.

I would like to walk the page table, so I have accessed the current->mm, but it gives NULL value.

我正在使用Linux内核3.9，但我不知道current-> mm怎么可能为零.

I'm working on linux kernel 3.9 and I don't understand how could current->mm is zero.

在这里我想念什么吗?

推荐答案

这表示您在内核线程中.

It means you are in a kernel thread.

在Linux中，内核线程没有mm结构.内核线程从前一个用户线程借用mm，并将其记录在active_mm中.因此，您应该改用active_mm .

In Linux, kernel threads have no mm struct. A kernel thread borrows the mm from the previous user thread and records it in active_mm. So you should use active_mm instead.

更多详细信息:

在/kernel/sched/core.c中，您可以找到以下代码:

in /kernel/sched/core.c you can find the following code:

static inline void
context_switch(struct rq *rq, struct task_struct *prev,
           struct task_struct *next)
{
    ...
    if (!mm) {
        next->active_mm = oldmm;
        atomic_inc(&oldmm->mm_count);
        enter_lazy_tlb(oldmm, next);
    } else
        switch_mm(oldmm, mm, next);
    ...
}

如果下一个线程没有mm(内核线程)，则调度程序将不会切换mm，而只会重用上一个线程的mm.

If the next thread has no mm (a kernel thread), the scheduler would not switch mm and just reuse the mm of the previous thread.

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