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

[Dan Williams]

On Tue, Sep 6, 2016 at 1:17 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 06 Sep 2016 09:49:41 -0700 Dan Williams <dan.j.williams@xxxxxxxxx> 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@xxxxxxxxxxxxxxx>
>> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Nilesh Choudhury <nilesh.choudhury@xxxxxxxxxx>
>> Reported-by: Kai Zhang <kai.ka.zhang@xxxxxxxxxx>
>> Reported-by: Toshi Kani <toshi.kani@xxxxxxx>
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
>
> 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.

> After fixing that,
>
> Acked-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

Thanks Andrew!