On Fri, Mar 11, 2016 at 01:40:20PM -0800, Dan Williams wrote: > On Fri, Mar 11, 2016 at 1:13 PM, Matthew Wilcox > <matthew.r.wilcox@xxxxxxxxx> wrote: > > By moving the flag bits to the bottom, we encourage commonality > > between SGs with pages and those using pfn_t. We can also then insert > > a pfn_t into a radix tree, as it uses the same two bits for indirect & > > exceptional indicators. > > It's not immediately clear to me what we gain with SG entry > commonality. The down side is that we lose the property that > pfn_to_pfn_t() is a nop. This was Dave's suggestion so that the > nominal case did not change the binary layout of a typical pfn. I understand that motivation! > Can we just bit swizzle a pfn_t on insertion/retrieval from the radix? Of course we *can*, but we end up doing more swizzling that way than we do this way. In the Brave New Future where we're storing pfn_t in the radix tree, on a page fault we find the pfn_t in the radix tree then we want to insert it into the page tables. So DAX would first have to convert the radix tree entry to a pfn_t, then the page table code has to convert the pfn_t into a pte/pmd/pud (which we currently do by converting a pfn_t to a pfn, then converting the pfn to a pte/pmd/pud, but I assume that either the compiler optimises that into a single conversion, or we'll add pfn_t_pte to each architecture in future if it's actually a problem). Much easier to look up a pfn_t in the radix tree and pass it directly to vm_insert_mixed(). If there's any part of the kernel that is doing a *lot* of conversion between pfn_t and pfn, that surely indicates a place in the kernel where we need to convert an interface from pfn to pfn_t. (It occurs to me we can make the code simpler on architectures that don't support PUDs. The PFN_HUGE bit is still available to distinguish between PMDs and PTEs, but we won't need to clear the bottom bit of the PFN if PFN_HUGE is set, since nobody can add a PUD pfn to the radix tree). -- 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>