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:
...

quoted

-		}
+	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,
-- 
John Hubbard
NVIDIA