Re: [PATCH v2 1/1] swap: shmem: remove SWAP_MAP_SHMEM

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Oct 1, 2024 at 7:06 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Tue, Oct 1, 2024 at 7:04 PM Nhat Pham <nphamcs@gmail.com> wrote:
> >
> > On Tue, Oct 1, 2024 at 6:58 PM Nhat Pham <nphamcs@gmail.com> wrote:
> > >
> > > On Tue, Oct 1, 2024 at 6:33 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> > >
> > > I was debating between WARN-ing here, and returning -ENOMEM and
> > > WARN-ing at shmem's callsite.
> > >
> > > My thinking is that if we return -ENOMEM here, it will work in the
> > > current setup, for both shmem and other callsites. However, in the
> > > future, if we add another user of swap_duplicate_nr(), this time
> > > without guaranteeing that we won't need continuation, I think it won't
> > > work unless we have the fallback logic in place as well:
> > >
> > > while (!err && __swap_duplicate(entry, 1, nr) == -ENOMEM)
> > > err = add_swap_count_continuation(entry, GFP_ATOMIC);
> >
> > Sorry, I accidentally sent out the email without completing my explanation :)
> >
> > Anyway, the point being, with the current implementation, any new user
> > would immediately hit a WARN and the implementer will know to check.
> >
> > Whereas if we return -ENOMEM in __swap_duplicate(), then I think we
> > would just hang, no? We only try to add swap count continuation to the
> > first entry only, which is not sufficient to fix the problem.
> >
> > I can probably whip up the fallback logic here, but it would be dead,
> > untestable code (as it has no users, and I cannot even conceive one to
> > test it). And the swap abstraction might render all of this moot
> > anyway.
>
> What I had in mind is not returning -ENOMEM at all, but something like
> -EOPNOTSUPP. The swap_duplicate_nr() will just return the error to the
> caller. All callers of swap_duplicate() and swap_duplicate_nr()
> currently check the error except shmem.

..and just to be extra clear, I meant WARN _and_ return -EOPNOTSUPP.