Re: [External] Re: [PATCH] fs/proc/kcore.c: add mmap interface

[zhoufeng]

在 2021/6/1 上午9:23, Andrew Morton 写道:
> On Wed, 26 May 2021 15:51:42 +0800 Feng zhou <zhoufeng.zf@xxxxxxxxxxxxx> wrote:
>
> > From: ZHOUFENG <zhoufeng.zf@xxxxxxxxxxxxx>
> >
> > When we do the kernel monitor, use the DRGN
> > (https://github.com/osandov/drgn) access to kernel data structures,
> > found that the system calls a lot. DRGN is implemented by reading
> > /proc/kcore. After looking at the kcore code, it is found that kcore
> > does not implement mmap, resulting in frequent context switching
> > triggered by read. Therefore, we want to add mmap interface to optimize
> > performance. Since vmalloc and module areas will change with allocation
> > and release, consistency cannot be guaranteed, so mmap interface only
> > maps KCORE_TEXT and KCORE_RAM.
> >
> > ...
> >
> > --- a/fs/proc/kcore.c
> > +++ b/fs/proc/kcore.c
> > @@ -573,11 +573,81 @@ static int release_kcore(struct inode *inode, struct file *file)
> >   	return 0;
> >   }
> >   
> > +static vm_fault_t mmap_kcore_fault(struct vm_fault *vmf)
> > +{
> > +	return VM_FAULT_SIGBUS;
> > +}
> > +
> > +static const struct vm_operations_struct kcore_mmap_ops = {
> > +	.fault = mmap_kcore_fault,
> > +};
> > +
> > +static int mmap_kcore(struct file *file, struct vm_area_struct *vma)
> > +{
> > +	size_t size = vma->vm_end - vma->vm_start;
> > +	u64 start, pfn;
> > +	int nphdr;
> > +	size_t data_offset;
> > +	size_t phdrs_len, notes_len;
> > +	struct kcore_list *m = NULL;
> > +	int ret = 0;
> > +
> > +	down_read(&kclist_lock);
> > +
> > +	get_kcore_size(&nphdr, &phdrs_len, &notes_len, &data_offset);
> > +
> > +	start = kc_offset_to_vaddr(((u64)vma->vm_pgoff << PAGE_SHIFT) -
> > +		((data_offset >> PAGE_SHIFT) << PAGE_SHIFT));
> > +
> > +	list_for_each_entry(m, &kclist_head, list) {
> > +		if (start >= m->addr && size <= m->size)
> > +			break;
> > +	}
> > +
> > +	if (&m->list == &kclist_head) {
> > +		ret = -EINVAL;
> > +		goto out;
> > +	}
> > +
> > +	if (vma->vm_flags & (VM_WRITE | VM_EXEC)) {
> > +		ret = -EPERM;
> > +		goto out;
> > +	}
> > +
> > +	vma->vm_flags &= ~(VM_MAYWRITE | VM_MAYEXEC);
> > +	vma->vm_flags |= VM_MIXEDMAP;
> > +	vma->vm_ops = &kcore_mmap_ops;
> > +
> > +	if (kern_addr_valid(start)) {
> > +		if (m->type == KCORE_RAM || m->type == KCORE_REMAP)
> > +			pfn = __pa(start) >> PAGE_SHIFT;
> > +		else if (m->type == KCORE_TEXT)
> > +			pfn = __pa_symbol(start) >> PAGE_SHIFT;
> > +		else {
> > +			ret = -EFAULT;
> > +			goto out;
> > +		}
> > +
> > +		if (remap_pfn_range(vma, vma->vm_start, pfn, size,
> > +				vma->vm_page_prot)) {
> > +			ret = -EAGAIN;
>
> EAGAIN seems a strange errno for this case.   The mmap manpage says
>
>         EAGAIN The file has been locked, or too much  memory  has  been  locked
>                (see setrlimit(2)).
>
>
> remap_pfn_range() already returns an errno - why not return whatever
> that code was?

  yes, that's a good idea.

> > +			goto out;
> > +		}
> > +	} else {
> > +		ret = -EFAULT;
> > +	}
> > +
> > +out:
> > +	up_read(&kclist_lock);
> > +	return ret;
> > +}
> > +
> >   static const struct proc_ops kcore_proc_ops = {
> >   	.proc_read	= read_kcore,
> >   	.proc_open	= open_kcore,
> >   	.proc_release	= release_kcore,
> >   	.proc_lseek	= default_llseek,
> > +	.proc_mmap	= mmap_kcore,
> >   };
> >   
> >   /* just remember that we have to update kcore */
>
> Otherwise looks OK to me.  Please update the changelog to reflect the
> discussion thus far and send a v2?
  OK, I am very happy to do so, I will send a v2 in two days.