On Tue, Sep 6, 2016 at 1:17 PM, Andrew Morton <[email protected]> wrote: > On Tue, 06 Sep 2016 09:49:41 -0700 Dan Williams <[email protected]> > 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 <[email protected]> >> Cc: Matthew Wilcox <[email protected]> >> Cc: Kirill A. Shutemov <[email protected]> >> Cc: Andrew Morton <[email protected]> >> Cc: Nilesh Choudhury <[email protected]> >> Reported-by: Kai Zhang <[email protected]> >> Reported-by: Toshi Kani <[email protected]> >> Cc: <[email protected]> >> Signed-off-by: Dan Williams <[email protected]> > > 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 <[email protected]> Thanks Andrew!
