Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Oct 20, 2023 at 3:02 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 19.10.23 21:53, Peter Xu wrote:
> > On Thu, Oct 19, 2023 at 05:41:01PM +0200, David Hildenbrand wrote:
> >> That's not my main point. It can easily become a maintenance burden without
> >> any real use cases yet that we are willing to support.
> >
> > That's why I requested a few times that we can discuss the complexity of
> > cross-mm support already here, and I'm all ears if I missed something on
> > the "maintenance burden" part..
> >
> > I started by listing what I think might be different, and we can easily
> > speedup single-mm with things like "if (ctx->mm != mm)" checks with
> > e.g. memcg, just like what this patch already did with pgtable depositions.
> >
> > We keep saying "maintenance burden" but we refuse to discuss what is that..
>
> Let's recap
>
> (1) We have person A up-streaming code written by person B, whereby B is
> not involved in the discussions nor seems to be active to maintain that
> code.
>
> Worse, the code that is getting up-streamed was originally based on a
> different kernel version that has significant differences in some key
> areas -- for example, page pinning, exclusive vs. shared.
>
> I claim that nobody here fully understands the code at hand (just look
> at the previous discussions), and reviewers have to sort out the mess
> that was created by the very way this stuff is getting upstreamed here.
>
> We're already struggling to get the single-mm case working correctly.
>
>
> (2) Cross-mm was not even announced anywhere nor mentioned which use it
> would have; I had to stumble over this while digging through the code.
> Further, is it even *tested*? AFAIKS in patch #3 no. Why do we have to
> make the life of reviewers harder by forcing them to review code that
> currently *nobody* on this earth needs?
>
>
> (3) You said "What else we can benefit from single mm?  One less mmap
> read lock, but probably that's all we can get;" and I presented two
> non-obvious issues. I did not even look any further because I really
> have better things to do than review complicated code without real use
> cases at hand. As I said "maybe that works as expected, I
> don't know and I have no time to spare on reviewing features with no
> real use cases)"; apparently I was right by just guessing that memcg
> handling is missing.
>
>
> The sub-feature in question (cross-mm) has no solid use cases; at this
> point I am not even convinced the use case you raised requires
> *userfaultfd*; for the purpose of moving a whole VMA worth of pages
> between two processes; I don't see the immediate need to get userfaultfd
> involved and move individual pages under page lock etc.

You make a compelling case against cross-mm support.
While I can't force Andrea to participate in upstreaming nor do I have
his background, keeping it simple, as you requested, is doable. That's
what I plan on doing by splitting the patch and I think we all agreed
to that. I'll also see if I can easily add a separate patch to test
cross-mm support.
I do apologize for the extra effort required from reviewers to cover
for the gaps in my patches. I'm doing my best to minimize that and I
really appreciate your time.

>
> >
> > I'll leave that to Suren and Lokesh to decide.  For me the worst case is
> > one more flag which might be confusing, which is not the end of the world..
> > Suren, you may need to work more thoroughly to remove cross-mm implications
> > if so, just like when renaming REMAP to MOVE.
>
> I'm asking myself why you are pushing so hard to include complexity
> "just because we can"; doesn't make any sense to me, honestly.
>
> Maybe you have some other real use cases that ultimately require
> userfaultfd for cross-mm that you cannot share?
>
> Will the world end when we have to use a separate flag so we can open
> this pandora's box when really required?
>
>
> Again, moving anon pages within a process is a known thing; we do that
> already via mremap; the only difference here really is, that we have to
> get the rmap right because we don't adjust VMAs. It's a shame we don't
> try to combine both code paths, maybe it's not easily possible like we
> did with mprotect vs. uffd-wp.

That's a good point. With cross-mm support baked in, the overlap was
not obvious to me. I'll see how much we can reuse from the mremap
path.

>
> Moving anon pages between process is currently only done via COW, where
> all things (page pinning, memcg, ...) have been figured out and are
> simply working as expected. Making uffd special by coding-up their own
> thing does not sound compelling to me.
>
>
> I am clearly against any unwarranted features+complexity. Again, I will
> stop arguing further, the whole thing of "include it just because we
> can" to avoid a flag (that we might never even see) doesn't make any
> sense to me and likely never will.
>
> The whole way this feature is getting upstreamed is just messed up IMHO
> and I the reasoning used in this thread to stick
> as-close-as-possible to some code person B wrote some years ago (e.g.,
> naming, sub-features) is far out of my comprehension.

I don't think staying as-close-as-possible to the original version was
the way I was driving this so far. At least that was not my conscious
intention. I'm open to further suggestions whenever it makes sense to
deviate from it.
Thanks,
Suren.

>
> --
> Cheers,
>
> David / dhildenb
>