Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

[David Hildenbrand]

On 04.10.23 01:39, Lokesh Gidra wrote:
> On Tue, Oct 3, 2023 at 11:26 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > On Tue, Oct 3, 2023 at 2:21 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> > > On Tue, Oct 03, 2023 at 11:08:07PM +0200, David Hildenbrand wrote:
> > > > Sorry I have to ask: has this ever been discussed on the list? I don't see
> > > > any pointers. If not, then probably the number of people that know about the
> > > > history can be counted with my two hands and that shouldn't be the basis for
> > > > making decisions.
> > >
> > > For example:
> > >
> > > https://lore.kernel.org/all/1425575884-2574-21-git-send-email-aarcange@xxxxxxxxxx/

Sorry, I had to process a family NMI the last couple of days.

> > There was another submission in 2019:
> > https://lore.kernel.org/all/cover.1547251023.git.blake.caldwell@xxxxxxxxxxxx/

It would be good to link them in the cover letter and shortly explain 
why that wasn't merged back then (if there was any reason).

> > Though both times it did not generate much discussion. I don't have a
> > strong preference though MOVE sounds more generic to me TBH (it
> > specifies the operation rather than REMAP which hints on how that
> > operation is carried out). But again, I'm fine either way.
>
> That's a good point. IMHO, if in future we want to have the fallback
> implemented, then MOVE would be a more appropriate name than REMAP.
>
> > As for UFFDIO_MOVE_ZERO_COPY_ONLY vs UFFDIO_MOVE_MODE_ALLOW_COPY, I
> > find it weird that the default (the most efficient/desired) mode of
> > operation needs a flag. I would prefer to have no flag initially and
> > add UFFDIO_MOVE_MODE_ALLOW_COPY or whatever name is more appropriate
> > when/if we ever need it. Makes sense?
>
> Agreed!

I agree. One could have UFFDIO_MOVE that is best-effort and documented 
like that, and a to-be-named future extension that always works but 
might be more expensive.


Ideally we'd have an interface that does not expose and/or rely on such 
low-level information and simply always works, but getting that would 
mean that we'd have to implement the fallback immediately ... so I guess 
we'll have to expose a best-effort interface first.

--
Cheers,

David / dhildenb