Re: [PATCH v4 1/6] libnvdimm/namespace: Make namespace size validation arch dependent

From: Dan Williams <dan.j.williams@intel.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: linux-nvdimm <linux-nvdimm@lists.01.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v4 1/6] libnvdimm/namespace: Make namespace size validation arch dependent
Date: Fri, 24 Jan 2020 10:25:59 -0800	[thread overview]
Message-ID: <CAPcyv4jPBfhC4t5+e2gxhzKfaLdQi_qTKfLEcXdo-MjTA5a+aw@mail.gmail.com> (raw)
In-Reply-To: <cb71528e-0ea9-cb56-6b51-ae7a5231ad54@linux.ibm.com>

On Fri, Jan 24, 2020 at 9:07 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 1/24/20 10:15 PM, Dan Williams wrote:
> > On Thu, Jan 23, 2020 at 11:34 PM Aneesh Kumar K.V
> > <aneesh.kumar@linux.ibm.com> wrote:
> >>
> >> On 1/24/20 11:27 AM, Dan Williams wrote:
> >>> On Mon, Jan 20, 2020 at 6:08 AM Aneesh Kumar K.V
> >>>
> >>
> >> ....
> >>
> >>>>
> >>>> +unsigned long arch_namespace_map_size(void)
> >>>> +{
> >>>> +       return PAGE_SIZE;
> >>>> +}
> >>>> +EXPORT_SYMBOL_GPL(arch_namespace_map_size);
> >>>> +
> >>>> +
> >>>>    static void __cpa_flush_all(void *arg)
> >>>>    {
> >>>>           unsigned long cache = (unsigned long)arg;
> >>>> diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
> >>>> index 9df091bd30ba..a3476dbd2656 100644
> >>>> --- a/include/linux/libnvdimm.h
> >>>> +++ b/include/linux/libnvdimm.h
> >>>> @@ -284,4 +284,5 @@ static inline void arch_invalidate_pmem(void *addr, size_t size)
> >>>>    }
> >>>>    #endif
> >>>>
> >>>> +unsigned long arch_namespace_map_size(void);
> >>>
> >>> This property is more generic than the nvdimm namespace mapping size,
> >>> it's more the fundamental remap granularity that the architecture
> >>> supports. So I would expect this to be defined in core header files.
> >>> Something like:
> >>>
> >>> diff --git a/include/linux/io.h b/include/linux/io.h
> >>> index a59834bc0a11..58b3b2091dbb 100644
> >>> --- a/include/linux/io.h
> >>> +++ b/include/linux/io.h
> >>> @@ -155,6 +155,13 @@ enum {
> >>>    void *memremap(resource_size_t offset, size_t size, unsigned long flags);
> >>>    void memunmap(void *addr);
> >>>
> >>> +#ifndef memremap_min_align
> >>> +static inline unsigned int memremap_min_align(void)
> >>> +{
> >>> +       return PAGE_SIZE;
> >>> +}
> >>> +#endif
> >>> +
> >>
> >>
> >> Should that be memremap_pages_min_align()?
> >
> > No, and on second look it needs to be a common value that results in
> > properly aligned / sized namespaces across architectures.
> >
> > What would it take for Power to make it's minimum mapping granularity
> > SUBSECTION_SIZE? The minute that the minimum alignment changes across
> > architectures we lose compatibility.
> >
> > The namespaces need to be sized such that the mode can be changed freely.
> >
>
> Linux on ppc64 with hash translation use just one page size for direct
> mapping and that is 16MB.

Ok, I think this means that the dream of SUBSECTION_SIZE being the
minimum compat alignment is dead, or at least a dream deferred.

Let's do this, change the name of this function to:

    memremap_compat_align()

...and define it to be the max() of all the alignment constraints that
the arch may require through either memremap(), or memremap_pages().
Then, teach ndctl to make its default alignment compatible by default,
16MiB, with an override to allow namespace creation with the current
architecture's memremap_compat_align(), exported via sysfs, if it
happens to be less then 16MiB. Finally, cross our fingers and hope
that Power remains the only arch that violates the SUBSECTION_SIZE
minimum value for memremap_compat_align().
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