Re: [PATCH v10 12/12] drivers/block/pmem: Map NVDIMM with ioremap_wt()

From: Dan Williams <dan.j.williams@intel.com>
To: Toshi Kani <toshi.kani@hp.com>
Cc: Borislav Petkov <bp@alien8.de>,
	Ross Zwisler <ross.zwisler@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-mm@kvack.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
	Juergen Gross <jgross@suse.com>,
	Stefan Bader <stefan.bader@canonical.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
	Yigal Korman <yigal@plexistor.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	"Elliott, Robert (Server Storage)" <Elliott@hp.com>,
	Luis Rodriguez <mcgrof@suse.com>, Christoph Hellwig <hch@lst.de>,
	Matthew Wilcox <willy@linux.intel.com>
Subject: Re: [PATCH v10 12/12] drivers/block/pmem: Map NVDIMM with ioremap_wt()
Date: Fri, 29 May 2015 12:34:32 -0700	[thread overview]
Message-ID: <CAPcyv4h82sjyXEz2GN=ttD3t-Z-owCyZTsfpkXc3D+KehwcAmg@mail.gmail.com> (raw)
In-Reply-To: <1432924340.23540.78.camel@misato.fc.hp.com>

On Fri, May 29, 2015 at 11:32 AM, Toshi Kani <toshi.kani@hp.com> wrote:
> On Fri, 2015-05-29 at 11:19 -0700, Dan Williams wrote:
>> On Fri, May 29, 2015 at 8:03 AM, Toshi Kani <toshi.kani@hp.com> wrote:
>> > On Fri, 2015-05-29 at 07:43 -0700, Dan Williams wrote:
>> >> On Fri, May 29, 2015 at 2:11 AM, Borislav Petkov <bp@alien8.de> wrote:
>> >> > On Wed, May 27, 2015 at 09:19:04AM -0600, Toshi Kani wrote:
>> >> >> The pmem driver maps NVDIMM with ioremap_nocache() as we cannot
>  :
>> >> >> -     pmem->virt_addr = ioremap_nocache(pmem->phys_addr, pmem->size);
>> >> >> +     pmem->virt_addr = ioremap_wt(pmem->phys_addr, pmem->size);
>> >> >>       if (!pmem->virt_addr)
>> >> >>               goto out_release_region;
>> >> >
>> >> > Dan, Ross, what about this one?
>> >> >
>> >> > ACK to pick it up as a temporary solution?
>> >>
>> >> I see that is_new_memtype_allowed() is updated to disallow some
>> >> combinations, but the manual seems to imply any mixing of memory types
>> >> is unsupported.  Which worries me even in the current code where we
>> >> have uncached mappings in the driver, and potentially cached DAX
>> >> mappings handed out to userspace.
>> >
>> > is_new_memtype_allowed() is not to allow some combinations of mixing of
>> > memory types.  When it is allowed, the requested type of ioremap_xxx()
>> > is changed to match with the existing map type, so that mixing of memory
>> > types does not happen.
>>
>> Yes, but now if the caller was expecting one memory type and gets
>> another one that is something I think the driver would want to know.
>> At a minimum I don't think we want to get emails about pmem driver
>> performance problems when someone's platform is silently degrading WB
>> to UC for example.
>
> The pmem driver creates an ioremap map to an NVDIMM range first.  So,
> there will be no conflict at this point, unless there is a conflicting
> driver claiming the same NVDIMM range.

Hmm, I thought it would be WB due to this comment in is_new_memtype_allowed()

        /*
         * PAT type is always WB for untracked ranges, so no need to check.
         */

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