Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user_pages()

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

On 4/18/23 17:36, Jason Gunthorpe wrote:
> On Tue, Apr 18, 2023 at 05:25:08PM +0100, Pavel Begunkov wrote:
>> On 4/17/23 13:56, Jason Gunthorpe wrote:
>>> On Sat, Apr 15, 2023 at 12:27:45AM +0100, Lorenzo Stoakes wrote:
>>>> Commit edd478269640 ("io_uring/rsrc: disallow multi-source reg buffers")
>>>> prevents io_pin_pages() from pinning pages spanning multiple VMAs with
>>>> permitted characteristics (anon/huge), requiring that all VMAs share the
>>>> same vm_file.
>>>
>>> That commmit doesn't really explain why io_uring is doing such a weird
>>> thing.
>>>
>>> What exactly is the problem with mixing struct pages from different
>>> files and why of all the GUP users does only io_uring need to care
>>> about this?
>>
>> Simply because it doesn't seem sane to mix and register buffers of
>> different "nature" as one.
> 
> That is not a good reason. Once things are converted to struct pages
> they don't need to care about their "nature"

Arguing purely about uapi, I do think it is. Even though it can be
passed down and a page is a page, Frankenstein's Monster mixing anon
pages, pages for io_uring queues, device shared memory, and what not
else doesn't seem right for uapi. I see keeping buffers as a single
entity in opposite to a set of random pages beneficial for the future.

And again, as for how it's internally done, I don't have any preference
whatsoever.

>> problem. We've been asked just recently to allow registering bufs
>> provided mapped by some specific driver, or there might be DMA mapped
>> memory in the future.
> 
> We already have GUP flags to deal with it, eg FOLL_PCI_P2PDMA
> 
>> Rejecting based on vmas might be too conservative, I agree and am all
>> for if someone can help to make it right.
> 
> It is GUP's problem to deal with this, not the callers.

Ok, that's even better for io_uring if the same can be achieved
just by passing flags.


> GUP is defined to return a list of normal CPU DRAM in struct page
> format. The caller doesn't care where or what this memory is, it is
> all interchangable - by API contract of GUP itself.
> 
> If you use FOLL_PCI_P2PDMA then the definition expands to allow struct
> pages that are MMIO.
> 
> In future, if someone invents new memory or new struct pages with
> special needs it is their job to ensure it is blocked from GUP - for
> *everyone*. eg how the PCI_P2PDMA was blocked from normal GUP.
> 
> io_uring is not special, there are many users of GUP, they all need to
> work consistently.

-- 
Pavel Begunkov