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

John Hubbard
2019-11-12T22:43:10Z

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,
--
John Hubbard
NVIDIA