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

Jens Axboe
2023-04-28T15:15:15Z

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.

--
Jens Axboe