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Date:   Wed, 19 Apr 2023 19:35:33 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>, Jens Axboe <axboe@kernel.dk>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.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()
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On Wed, Apr 19, 2023 at 03:24:55PM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 19, 2023 at 07:23:00PM +0100, Matthew Wilcox wrote:
> > On Wed, Apr 19, 2023 at 07:18:26PM +0100, Lorenzo Stoakes wrote:
> > > 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.
> > 
> > I don't even understand why Pavel wanted to make this change.  The
> > commit log really doesn't say.
> > 
> > commit edd478269640
> > Author: Pavel Begunkov <asml.silence@gmail.com>
> > Date:   Wed Feb 22 14:36:48 2023 +0000
> > 
> >     io_uring/rsrc: disallow multi-source reg buffers
> > 
> >     If two or more mappings go back to back to each other they can be passed
> >     into io_uring to be registered as a single registered buffer. That would
> >     even work if mappings came from different sources, e.g. it's possible to
> >     mix in this way anon pages and pages from shmem or hugetlb. That is not
> >     a problem but it'd rather be less prone if we forbid such mixing.
> > 
> >     Cc: <stable@vger.kernel.org>
> >     Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> >     Signed-off-by: Jens Axboe <axboe@kernel.dk>
> > 
> > It even says "That is not a problem"!  So why was this patch merged
> > if it's not fixing a problem?
> > 
> > It's now standing in the way of an actual cleanup.  So why don't we
> > revert it?  There must be more to it than this ...
> 
> https://lore.kernel.org/all/61ded378-51a8-1dcb-b631-fda1903248a9@gmail.com/

So um, it's disallowed because Pavel couldn't understand why it
should be allowed?  This gets less and less convincing.

FWIW, what I was suggesting was that we should have a FOLL_SINGLE_VMA
flag, which would use our shiny new VMA lock infrastructure to look
up and lock _one_ VMA instead of having the caller take the mmap_lock.
Passing that flag would be a tighter restriction that Pavel implemented,
but would certainly relieve some of his mental load.

By the way, even if all pages are from the same VMA, they may still be a
mixture of anon and file pages; think a MAP_PRIVATE of a file when
only some pages have been written to.  Or an anon MAP_SHARED which is
accessible by a child process.