Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Mar 13, 2019 at 8:45 PM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
[..]
> >> Now w.r.t to failures, can device-dax do an opportunistic huge page
> >> usage?
> >
> > device-dax explicitly disclaims the ability to do opportunistic mappings.
> >
> >> I haven't looked at the device-dax details fully yet. Do we make the
> >> assumption of the mapping page size as a format w.r.t device-dax? Is that
> >> derived from nd_pfn->align value?
> >
> > Correct.
> >
> >>
> >> Here is what I am working on:
> >> 1) If the platform doesn't support huge page and if the device superblock
> >> indicated that it was created with huge page support, we fail the device
> >> init.
> >
> > Ok.
> >
> >> 2) Now if we are creating a new namespace without huge page support in
> >> the platform, then we force the align details to PAGE_SIZE. In such a
> >> configuration when handling dax fault even with THP enabled during
> >> the build, we should not try to use hugepage. This I think we can
> >> achieve by using TRANSPARENT_HUGEPAEG_DAX_FLAG.
> >
> > How is this dynamic property communicated to the guest?
>
> via device tree on powerpc. We have a device tree node indicating
> supported page sizes.

Ah, ok, yeah let's plumb that straight to the device-dax driver and
leave out the interaction / interpretation of the thp-enabled flags.

>
> >
> >>
> >> Also even if the user decided to not use THP, by
> >> echo "never" > transparent_hugepage/enabled , we should continue to map
> >> dax fault using huge page on platforms that can support huge pages.
> >>
> >> This still doesn't cover the details of a device-dax created with
> >> PAGE_SIZE align later booted with a kernel that can do hugepage dax.How
> >> should we handle that? That makes me think, this should be a VMA flag
> >> which got derived from device config? May be use VM_HUGEPAGE to indicate
> >> if device should use a hugepage mapping or not?
> >
> > device-dax configured with PAGE_SIZE always gets PAGE_SIZE mappings.
>
> Now what will be page size used for mapping vmemmap?

That's up to the architecture's vmemmap_populate() implementation.

> Architectures
> possibly will use PMD_SIZE mapping if supported for vmemmap. Now a
> device-dax with struct page in the device will have pfn reserve area aligned
> to PAGE_SIZE with the above example? We can't map that using
> PMD_SIZE page size?

IIUC, that's a different alignment. Currently that's handled by
padding the reservation area up to a section (128MB on x86) boundary,
but I'm working on patches to allow sub-section sized ranges to be
mapped.

Now, that said, I expect there may be bugs lurking in the
implementation if PAGE_SIZE changes from one boot to the next simply
because I've never tested that.

I think this also indicates that the section padding logic can't be
removed until all arch vmemmap_populate() implementations understand
the sub-section case.