Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in pin_user_pages()

[Pavel Begunkov <asml.silence@gmail.com>]

On 4/19/23 19:35, Matthew Wilcox wrote:
> On Wed, Apr 19, 2023 at 03:24:55PM -0300, Jason Gunthorpe wrote:
>> On Wed, Apr 19, 2023 at 07:23:00PM +0100, Matthew Wilcox wrote:
>>> On Wed, Apr 19, 2023 at 07:18:26PM +0100, Lorenzo Stoakes wrote:
>>>> So even if I did the FOLL_ALLOW_BROKEN_FILE_MAPPING patch series first, I
>>>> would still need to come along and delete a bunch of your code
>>>> afterwards. And unfortunately Pavel's recent change which insists on not
>>>> having different vm_file's across VMAs for the buffer would have to be
>>>> reverted so I expect it might not be entirely without discussion.
>>>
>>> I don't even understand why Pavel wanted to make this change.  The
>>> commit log really doesn't say.
>>>
>>> commit edd478269640
>>> Author: Pavel Begunkov <asml.silence@gmail.com>
>>> Date:   Wed Feb 22 14:36:48 2023 +0000
>>>
>>>      io_uring/rsrc: disallow multi-source reg buffers
>>>
>>>      If two or more mappings go back to back to each other they can be passed
>>>      into io_uring to be registered as a single registered buffer. That would
>>>      even work if mappings came from different sources, e.g. it's possible to
>>>      mix in this way anon pages and pages from shmem or hugetlb. That is not
>>>      a problem but it'd rather be less prone if we forbid such mixing.
>>>
>>>      Cc: <stable@vger.kernel.org>
>>>      Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>>      Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>
>>> It even says "That is not a problem"!  So why was this patch merged
>>> if it's not fixing a problem?
>>>
>>> It's now standing in the way of an actual cleanup.  So why don't we
>>> revert it?  There must be more to it than this ...
>>
>> https://lore.kernel.org/all/61ded378-51a8-1dcb-b631-fda1903248a9@gmail.com/
> 
> So um, it's disallowed because Pavel couldn't understand why it
> should be allowed?  This gets less and less convincing.

Excuse me? I'm really sorry you "couldn't understand" the explanation
as it has probably been too much of a "mental load", but let me try to
elaborate.

Because it's currently limited what can be registered, it's indeed not
a big deal, but that will most certainly change, and I usually and
apparently nonsensically prefer to tighten things up _before_ it becomes
a problem. And again, taking a random set of buffers created for
different purposes and registering it as a single entity is IMHO not a
sane approach.

Take p2pdma for instance, if would have been passed without intermixing
there might not have been is_pci_p2pdma_page()/etc. for every single page
in a bvec. That's why in general, it won't change for p2pdma but there
might be other cases in the future.


> FWIW, what I was suggesting was that we should have a FOLL_SINGLE_VMA
> flag, which would use our shiny new VMA lock infrastructure to look
> up and lock _one_ VMA instead of having the caller take the mmap_lock.
> Passing that flag would be a tighter restriction that Pavel implemented,
> but would certainly relieve some of his mental load.
> 
> By the way, even if all pages are from the same VMA, they may still be a
> mixture of anon and file pages; think a MAP_PRIVATE of a file when
> only some pages have been written to.  Or an anon MAP_SHARED which is
> accessible by a child process.

-- 
Pavel Begunkov