On Wed, Dec 2, 2015 at 9:43 AM, Toshi Kani <toshi.kani@xxxxxxx> wrote: > Oh, I see. I will setup the memmap array and run the tests again. > > But, why does the PMD mapping depend on the memmap array? We have observed > major performance improvement with PMD. This feature should always be enabled > with DAX regardless of the option to allocate the memmap array. > Several factors drove this decision, I'm open to considering alternatives but here's the reasoning: 1/ DAX pmd mappings caused crashes in the get_user_pages path leading to commit e82c9ed41e8 "dax: disable pmd mappings". The reason pte mappings don't crash and instead trigger -EFAULT is due to the _PAGE_SPECIAL pte bit. 2/ To enable get_user_pages for DAX, in both the page and huge-page case, we need a new pte bit _PAGE_DEVMAP. 3/ Given the pte bits are hard to come I'm assuming we won't get two, i.e. both _PAGE_DEVMAP and a new _PAGE_SPECIAL for pmds. Even if we could get a _PAGE_SPECIAL for pmds I'm not in favor of pursuing it. End result is that DAX pmd mappings must be fully enabled through the get_user_pages paths with _PAGE_DEVMAP or turned off completely. In general I think the "page less" DAX implementation was a good starting point, but we need to shift to page-backed by default until we can teach more of the kernel to operate on bare pfns. That "default" will need to be enforced by userspace tooling. -- 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>