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 15:24:55 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>, Jens Axboe <axboe@...nel.dk>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.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 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/

Jason