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

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Date:   Wed, 20 Mar 2019 08:34:20 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Jan Kara <jack@...e.cz>, linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Ross Zwisler <zwisler@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 2/2] mm/dax: Don't enable huge dax mapping by default

On Wed, Mar 20, 2019 at 1:09 AM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> wrote:
>
> Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com> writes:
>
> > Dan Williams <dan.j.williams@...el.com> writes:
> >
> >>
> >>> 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.
> >
> > I am missing something w.r.t code. The below code align that using nd_pfn->align
> >
> >       if (nd_pfn->mode == PFN_MODE_PMEM) {
> >               unsigned long memmap_size;
> >
> >               /*
> >                * vmemmap_populate_hugepages() allocates the memmap array in
> >                * HPAGE_SIZE chunks.
> >                */
> >               memmap_size = ALIGN(64 * npfns, HPAGE_SIZE);
> >               offset = ALIGN(start + SZ_8K + memmap_size + dax_label_reserve,
> >                               nd_pfn->align) - start;
> >       }
> >
> > IIUC that is finding the offset where to put vmemmap start. And that has
> > to be aligned to the page size with which we may end up mapping vmemmap
> > area right?

Right, that's the physical offset of where the vmemmap ends, and the
memory to be mapped begins.

> > Yes we find the npfns by aligning up using PAGES_PER_SECTION. But that
> > is to compute howmany pfns we should map for this pfn dev right?
> >
>
> Also i guess those 4K assumptions there is wrong?

Yes, I think to support non-4K-PAGE_SIZE systems the 'pfn' metadata
needs to be revved and the PAGE_SIZE needs to be recorded in the
info-block.