Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages

[John Hubbard <jhubbard@nvidia.com>]

On 8/27/22 17:39, Al Viro wrote:
> On Sun, Aug 28, 2022 at 01:38:57AM +0100, Al Viro wrote:
>> On Sat, Aug 27, 2022 at 04:55:18PM -0700, John Hubbard wrote:
>>> On 8/27/22 15:48, Al Viro wrote:
>>>> On Sat, Aug 27, 2022 at 01:36:06AM -0700, John Hubbard wrote:
>>>>> Convert the NFS Direct IO layer to use pin_user_pages_fast() and
>>>>> unpin_user_page(), instead of get_user_pages_fast() and put_page().
>>>>
>>>> Again, this stuff can be hit with ITER_BVEC iterators
>>>>
>>>>> -		result = iov_iter_get_pages_alloc2(iter, &pagevec,
>>>>> +		result = dio_w_iov_iter_pin_pages_alloc(iter, &pagevec,
>>>>>  						  rsize, &pgbase);
>>>>
>>>> and this will break on those.
>>>
>>> If anyone has an example handy, of a user space program that leads
>>> to this situation (O_DIRECT with ITER_BVEC), it would really help
>>> me reach enlightenment a lot quicker in this area. :)
>>
>> Er...  splice(2) to O_DIRECT-opened file on e.g. ext4?  Or
>> sendfile(2) to the same, for that matter...
> 
> s/ext4/nfs/ to hit this particular codepath, obviously.

OK, I have a solution to this that's pretty easy:

1) Get rid of the user_backed_iter(i) check in
dio_w_iov_iter_pin_pages() and dio_w_iov_iter_pin_pages_alloc(), and

2) At the call sites, match up the unpin calls appropriately.

...and apply a similar fix for the fuse conversion patch.

However, the core block/bio conversion in patch 4 still does depend upon
a key assumption, which I got from a 2019 email discussion with
Christoph Hellwig and others here [1], which says:

    "All pages released by bio_release_pages should come from
     get_get_user_pages...".

I really hope that still holds true. Otherwise this whole thing is in
trouble.



[1] https://lore.kernel.org/kvm/20190724053053.GA18330@infradead.org/

thanks,

-- 
John Hubbard
NVIDIA