linux-kernel - Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user

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Date:   Mon, 17 Apr 2023 10:26:09 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>,
        io-uring@...r.kernel.org
Subject: Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on
 pin_user_pages()

On Mon, Apr 17, 2023 at 02:19:16PM +0100, Lorenzo Stoakes wrote:

> > I'd rather see something like FOLL_ALLOW_BROKEN_FILE_MAPPINGS than
> > io_uring open coding this kind of stuff.
> >
> 
> How would the semantics of this work? What is broken? It is a little
> frustrating that we have FOLL_ANON but hugetlb as an outlying case, adding
> FOLL_ANON_OR_HUGETLB was another consideration...

It says "historically this user has accepted file backed pages and we
we think there may actually be users doing that, so don't break the
uABI"

Without the flag GUP would refuse to return file backed pages that can
trigger kernel crashes or data corruption.

Eg we'd want most places to not specify the flag and the few that do
to have some justification.

We should consdier removing FOLL_ANON, I'm not sure it really makes
sense these days for what proc is doing with it. All that proc stuff
could likely be turned into a kthread_use_mm() and a simple
copy_to/from user?

I suspect that eliminates the need to check for FOLL_ANON?

Jason