Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 19 Apr 2023 19:50:57 +0100
From:   Lorenzo Stoakes <lstoakes@gmail.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org
Subject: Re: [PATCH v4 4/6] io_uring: rsrc: avoid use of vmas parameter in
 pin_user_pages()
Message-ID: <0a41fd0e-39ce-48a4-b47b-53cf4cbb050b@lucifer.local>
References: <cover.1681831798.git.lstoakes@gmail.com>
 <956f4fc2204f23e4c00e9602ded80cb4e7b5df9b.1681831798.git.lstoakes@gmail.com>
 <936e8f52-00be-6721-cb3e-42338f2ecc2f@kernel.dk>
 <c2e22383-43ee-5cf0-9dc7-7cd05d01ecfb@kernel.dk>
 <f82b9025-a586-44c7-9941-8140c04a4ccc@lucifer.local>
 <69f48cc6-8fc6-0c49-5a79-6c7d248e4ad5@kernel.dk>
 <bec03e0f-a0f9-43c3-870b-be406ca848b9@lucifer.local>
 <8af483d2-0d3d-5ece-fb1d-a3654411752b@kernel.dk>
 <d601ca0c-d9b8-4e5d-a047-98f2d1c65eb9@lucifer.local>
 <ZEAxW/yR3LCNSmjT@nvidia.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZEAxW/yR3LCNSmjT@nvidia.com>
Precedence: bulk

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 :)