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

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Date:   Thu, 20 Apr 2023 14:37:57 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, Jason Gunthorpe <jgg@...dia.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.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 4/19/23 21:15, Jens Axboe wrote:
> On 4/19/23 12:24?PM, 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@...il.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@...r.kernel.org>
>>>      Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>>>      Signed-off-by: Jens Axboe <axboe@...nel.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/
> 
> Let's just kill that patch that, I can add a revert for 6.4. I had
> forgotten about that patch and guess I didn't realize that most of the
> issue do in fact just stem from that.

Well, we're now trading uapi sanity for cleanups, I don't know
what I should take out of it.

-- 
Pavel Begunkov