On Wed, Sep 7, 2016 at 12:39 PM, Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote: > 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@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. > > 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. Yes, indeed! In fact I already have that re-write getting 0day coverage before posting. It occurred to me while re-writing the changelog per Andrew's prompting. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>