linux-kernel - Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user

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Date:   Tue, 18 Apr 2023 15:19:32 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org
Subject: Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on
 pin_user_pages()

On Tue, Apr 18, 2023 at 06:25:24PM +0100, Pavel Begunkov wrote:
> 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.

Again, it is not up to io_uring to make this choice. We have GUP as
part of our uAPI all over the place, GUP decides how it works, not
random different ideas all over the place.

We don't have these kinds of restrictions for O_DIRECT, for instance.

There should be consistency in the uAPI across everything.

Jason