Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

On 11/12/19 12:43 PM, Jason Gunthorpe wrote: ...

...

• }
  

• ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,

• 		    page, vmas, NULL);
  

• /*

• * The lifetime of a vaddr_get_pfn() page pin is
  

• * userspace-controlled. In the fs-dax case this could
  

• * lead to indefinite stalls in filesystem operations.
  

• * Disallow attempts to pin fs-dax pages via this
  

• * interface.
  

• */
  

• if (ret > 0 && vma_is_fsdax(vmas[0])) {

• ret = -EOPNOTSUPP;
  

• put_page(page[0]);
  

  }

AFAIK this chunk is redundant now as it is some hack to emulate FOLL_LONGTERM? So vmas can be deleted too.

Let me first make sure I understand what Dan has in mind for the vma checking, in the other thread...

Also unclear why this function has this:

    up_read(&mm->mmap_sem);

    if (ret == 1) {
            *pfn = page_to_pfn(page[0]);
            return 0;
    }

    down_read(&mm->mmap_sem);

Yes, that's really odd. It's not good to release and retake the lock anyway in general (without re-checking things), and certainly it is not required to release mmap_sem in order to call page_to_pfn().

I've removed that up_read()/down_read() pair, for v4.

thanks,