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

[Peter Xu]

James,

On Wed, Nov 16, 2022 at 03:30:00PM -0800, James Houghton wrote:
> 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.

Sorry to get back to this very late - I know this has been discussed since
the very early stage of the feature, but is there any reasoning behind?

When I start to think seriously on applying this to process snapshot with
uffd-wp I found that the minor mode trick won't easily play - normally
that's a case where all the pages were there mapped huge, but when the app
wants UFFDIO_WRITEPROTECT it may want to remap the huge pages into smaller
pages, probably some size that the user can specify.  It'll be non-trivial
to enable HGM during that phase using MINOR mode because in that case the
pages are all mapped.

For the long term, I am just still worried the current interface is still
not as flexible.

-- 
Peter Xu