Re: [RFC PATCH v2 33/47] userfaultfd: add UFFD_FEATURE_MINOR_HUGETLBFS_HGM

[James Houghton]

On Wed, Nov 16, 2022 at 2:28 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Fri, Oct 21, 2022 at 04:36:49PM +0000, James Houghton wrote:
> > Userspace must provide this new feature when it calls UFFDIO_API to
> > enable HGM. Userspace can check if the feature exists in
> > uffdio_api.features, and if it does not exist, the kernel does not
> > support and therefore did not enable HGM.
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
>
> It's still slightly a pity that this can only be enabled by an uffd context
> plus a minor fault, so generic hugetlb users cannot directly leverage this.

The idea here is that, for applications that can conceivably benefit
from HGM, we have a mechanism for enabling it for that application. So
this patch creates that mechanism for userfaultfd/UFFDIO_CONTINUE. I
prefer this approach over something more general like MADV_ENABLE_HGM
or something.

For hwpoison, HGM will be automatically enabled, but that isn't
implemented in this series. We could also extend MADV_DONTNEED to do
high-granularity unmapping in some way, but that also isn't attempted
here. I'm sure that if there are other cases where HGM may be useful,
we can add/change some uapi to make it possible to take advantage HGM.

- James

>
> The patch itself looks good.
>
> --
> Peter Xu
>