linux-kernel - Re: [PATCH v5 4/6] io_uring: rsrc: delegate VMA file-backed check to GUP

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Date:   Tue, 16 May 2023 07:19:52 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     David Hildenbrand <david@...hat.com>,
        Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v5 4/6] io_uring: rsrc: delegate VMA file-backed check to
 GUP

On 5/16/23 2:25?AM, David Hildenbrand wrote:
> On 15.05.23 21:55, Jens Axboe wrote:
>> On 5/14/23 3:26?PM, Lorenzo Stoakes wrote:
>>> Now that the GUP explicitly checks FOLL_LONGTERM pin_user_pages() for
>>> broken file-backed mappings in "mm/gup: disallow FOLL_LONGTERM GUP-nonfast
>>> writing to file-backed mappings", there is no need to explicitly check VMAs
>>> for this condition, so simply remove this logic from io_uring altogether.
>>
>> Don't have the prerequisite patch handy (not in mainline yet), but if it
>> just moves the check, then:
>>
>> Reviewed-by: Jens Axboe <axboe@...nel.dk>
>>
> 
> Jens, please see my note regarding iouring:
> 
> https://lore.kernel.org/bpf/6e96358e-bcb5-cc36-18c3-ec5153867b9a@redhat.com/
> 
> With this patch, MAP_PRIVATE will work as expected (2), but there will
> be a change in return code handling (1) that we might have to document
> in the man page.

I think documenting that newer kernels will return -EFAULT rather than
-EOPNOTSUPP should be fine. It's not a new failure case, just a
different error value for an already failing case. Should be fine with
just a doc update. Will do that now.

-- 
Jens Axboe