linux-kernel - Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in pin_user

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Date:   Wed, 19 Apr 2023 19:50:57 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jens Axboe <axboe@...nel.dk>, 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>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org
Subject: Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in
 pin_user_pages()

On Wed, Apr 19, 2023 at 03:22:19PM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 19, 2023 at 07:18:26PM +0100, Lorenzo Stoakes wrote:
>
> > I'd also argue that we are doing things right with this patch series as-is,
> > io_uring is the only sticking point because, believe it or not, it is the
> > only place in the kernel that uses multiple vmas (it has been interesting
> > to get a view on GUP use as a whole here).
>
> I would say io_uring is the only place trying to open-code bug fixes
> for MM problems :\ As Jens says, these sorts of temporary work arounds become
> lingering problems that nobody wants to fix properly.
>
> > 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.
>
> Yeah, that should just be reverted.
>
> > However, if you really do feel that you can't accept this change as-is, I
> > can put this series on hold and look at FOLL_ALLOW_BROKEN_FILE_MAPPING and
> > we can return to this afterwards.
>
> It is probably not as bad as you think, I suspect only RDMA really
> wants to set the flag. Maybe something in media too, maybe.
>
> Then again that stuff doesn't work so incredibly badly maybe there
> really is no user of it and we should just block it completely.
>
> Jason

OK in this case I think we're all agreed that it's best to do this first
then revisit this series afterwards. I will switch to working on this!

And I will make sure to cc- everyone in :)