Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Fri, Apr 28, 2023 at 09:15:08AM -0600, Jens Axboe wrote:
> On 4/28/23 9:13?AM, David Hildenbrand wrote:
> >>> I know, Jason und John will disagree, but I don't think we want to be very
> >>> careful with changing the default.
> >>>
> >>> Sure, we could warn, or convert individual users using a flag (io_uring).
> >>> But maybe we should invest more energy on a fix?
> >>
> >> This is proactively blocking a cleanup (eliminating vmas) that I believe
> >> will be useful in moving things forward. I am not against an opt-in option
> >> (I have been responding to community feedback in adapting my approach),
> >> which is the way I implemented it all the way back then :)
> >
> > There are alternatives: just use a flag as Jason initially suggested
> > and use that in io_uring code. Then, you can also bail out on the
> > GUP-fast path as "cannot support it right now, never do GUP-fast".
>
> Since I've seen this brougth up a few times, what's the issue on the
> io_uring side? We already dropped the special vma checking, it's in -git
> right. Hence I don't believe there are any special cases left for
> io_uring at all, and we certainly don't allow real file backings either,
> never have done.

The purpose from my perspective is being able to have GUP perform the 'is the
file-backed mapping sane to GUP' check rather than you having to open code
it. There is nothing special beyond that.

Personally I think the best solution is an opt-in FOLL_SAFE_WRITE_FILE flag or
such that you call and drop the vma check you have.

That way we don't risk breaking anything, the vmas patch series can unblock, and
you don't have to have raw mm code in your bit :)

>
> --
> Jens Axboe
>