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From: Pavel Begunkov <[email protected]>
To: Jason Gunthorpe <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>,
	[email protected], [email protected],
	Andrew Morton <[email protected]>,
	Matthew Wilcox <[email protected]>,
	David Hildenbrand <[email protected]>,
	Jens Axboe <[email protected]>,
	[email protected]
Subject: Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user_pages()
Date: Tue, 18 Apr 2023 18:25:24 +0100	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <ZD7HGbdBt1XqIDX/@nvidia.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

  reply	other threads:[~2023-04-18 17:26 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <[email protected]>
2023-04-14 23:27 ` [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user_pages() Lorenzo Stoakes
2023-04-17 12:56   ` Jason Gunthorpe
2023-04-17 13:19     ` Lorenzo Stoakes
2023-04-17 13:26       ` Jason Gunthorpe
2023-04-17 14:00         ` Lorenzo Stoakes
2023-04-17 14:15           ` Jason Gunthorpe
2023-04-17 15:20             ` Lorenzo Stoakes
2023-04-17 19:00         ` Lorenzo Stoakes
2023-04-17 19:24           ` Jason Gunthorpe
2023-04-17 19:45             ` Lorenzo Stoakes
2023-04-18 16:25     ` Pavel Begunkov
2023-04-18 16:35       ` Pavel Begunkov
2023-04-18 16:36       ` Jason Gunthorpe
2023-04-18 17:25         ` Pavel Begunkov [this message]
2023-04-18 18:19           ` Jason Gunthorpe
2023-04-14 23:27 ` [PATCH 6/7] mm/gup: remove vmas parameter from pin_user_pages() Lorenzo Stoakes

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