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

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Message-ID: <CAPcyv4hMzVuOYzy2tTq-my8Z1y+X6Ug-fyObpKTxVU44p5rBZw@mail.gmail.com>
Date:   Wed, 20 Mar 2019 13:57:25 -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 8:34 AM Dan Williams <dan.j.williams@...el.com> wrote:
>
> 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.

How often does a system change page-size. Is it fixed or do
environment change it from one boot to the next? I'm thinking through
the behavior of what do when the recorded PAGE_SIZE in the info-block
does not match the current system page size. The simplest option is to
just fail the device and require it to be reconfigured. Is that
acceptable?