Re: [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user_pages()

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Apr 17, 2023 at 04:24:04PM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 17, 2023 at 08:00:48PM +0100, Lorenzo Stoakes wrote:
>
> > So I don't think this route is plausible unless you were thinking of
> > somehow offloading to a thread?
>
> ah, fair enough
>
> > In any case, if we institute the FOLL_ALLOW_BROKEN_FILE_MAPPINGS flag we
> > can just drop FOLL_ANON altogether right, as this will be implied and
> > hugetlb should work here too?
>
> Well.. no, as I said read-only access to the pages works fine, so GUP
> should not block that. It is only write that has issues
>
> > Separately, I find the semantics of access_remote_vm() kind of weird, and
> > with a possible mmap_lock-free future it does make me wonder whether
> > something better could be done there.
>
> Yes, it is very weird, kthread_use_mm is much nicer.
>
> > (Section where I sound like I might be going mad) Perhaps having some means
> > of context switching into the kernel portion of the remote process as if
> > were a system call or soft interrupt handler and having that actually do
> > the uaccess operation could be useful here?
>
> This is the kthread_use_mm() approach, that is basically what it
> does. You are suggesting to extend it to kthreads that already have a
> process attached...

Yeah, I wonder how plausible this is as we could in theory simply eliminate
these remote cases altogether which could be relatively efficient if we
could find a way to batch up operations.

>
> access_remote_vm is basically copy_to/from_user built using kmap and
> GUP.
>
> even a simple step of localizing FOLL_ANON to __access_remote_vm,
> since it must have the VMA nyhow, would be an improvement.

This is used from places where this flag might not be set though,
e.g. acess_process_vm() and ptrace.

However, access_remote_vm() is only used by the proc stuff, so I will spin
up a patch to move this function and treat it as a helper which sets
FOLL_ANON.

>
> Jason