On Wed, Dec 2, 2015 at 11:26 AM, Toshi Kani <toshi.kani@xxxxxxx> wrote: > On Wed, 2015-12-02 at 10:06 -0800, Dan Williams wrote: >> On Wed, Dec 2, 2015 at 9:01 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: >> > 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. >> >> Actually, Dave says they aren't that hard to come by for pmds, so we >> could go add _PMD_SPECIAL if we really wanted to support the limited >> page-less DAX-pmd case. >> >> But I'm still of the opinion that we run away from the page-less case >> until it can be made a full class citizen with O_DIRECT for pfn >> support. > > I may be missing something, but per vm_normal_page(), I think _PAGE_SPECIAL can > be substituted by the following check when we do not have the memmap. > > if ((vma->vm_flags & VM_PFNMAP) || > ((vma->vm_flags & VM_MIXEDMAP) && (!pfn_valid(pfn)))) { > > This is what I did in this patch for follow_trans_huge_pmd(), although I missed > the pfn_valid() check. That works for __get_user_pages but not __get_user_pages_fast where we don't have access to the vma. -- 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>