Re: [PATCH 4/5] mm: fix cache mode of dax pmd mappings

["Kani, Toshimitsu" <toshi.kani-ZPxbGqLxI0U@public.gmane.org>]

On Tue, 2016-09-06 at 14:52 -0700, Dan Williams wrote:
> On Tue, Sep 6, 2016 at 1:17 PM, Andrew Morton <akpm@linux-foundation.
> org> wrote:
> > 
> > On Tue, 06 Sep 2016 09:49:41 -0700 Dan Williams <dan.j.williams@int
> > el.com> wrote:
> > 
> > > 
> > > track_pfn_insert() is marking dax mappings as uncacheable.
> > > 
> > > It is used to keep mappings attributes consistent across a
> > > remapped range. However, since dax regions are never registered
> > > via track_pfn_remap(), the caching mode lookup for dax pfns
> > > always returns _PAGE_CACHE_MODE_UC.  We do not use
> > > track_pfn_insert() in the dax-pte path, and we always want to use
> > > the pgprot of the vma itself, so drop this call.
> > > 
> > > Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> > > Cc: Matthew Wilcox <mawilcox@microsoft.com>
> > > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Nilesh Choudhury <nilesh.choudhury@oracle.com>
> > > Reported-by: Kai Zhang <kai.ka.zhang@oracle.com>
> > > Reported-by: Toshi Kani <toshi.kani@hpe.com>
> > > Cc: <stable@vger.kernel.org>
> > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > 
> > Changelog fails to explain the user-visible effects of the
> > patch.  The stable maintainer(s) will look at this and wonder "ytf
> > was I sent this".
> 
> True, I'll change it to this:
> 
> track_pfn_insert() is marking dax mappings as uncacheable rendering
> them impractical for application usage.  DAX-pte mappings are cached
> and the goal of establishing DAX-pmd mappings is to attain more
> performance, not dramatically less (3 orders of magnitude).
> 
> Deleting the call to track_pfn_insert() in vmf_insert_pfn_pmd() lets
> the default pgprot (write-back cache enabled) from the vma be used
> for the mapping which yields the expected performance improvement
> over DAX-pte mappings.
> 
> track_pfn_insert() is meant to keep the cache mode for a given range
> synchronized across different users of remap_pfn_range() and
> vm_insert_pfn_prot().  DAX uses neither of those mapping methods, and
> the pmem driver is already marking its memory ranges as write-back
> cache enabled.  So, removing the call to track_pfn_insert() leaves
> the kernel no worse off than the current situation where a user could
> map the range via /dev/mem with an incompatible cache mode compared
> to the driver.

I think devm_memremap_pages() should call reserve_memtype() on x86 to
keep it consistent with devm_memremap() on this regard.  We may need an
arch stub for reserve_memtype(), though.  Then, track_pfn_insert()
should have no issue in this case.

Thanks,
-Toshi
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