On Sat, Mar 12, 2016 at 10:30 AM, Matthew Wilcox <willy@xxxxxxxxxxxxxxx> wrote: > 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. So this is dependent on where pfn_t gets pushed in the future. For example, if we revive using a pfn_t in a bio then I think the pfn_to_pfn_t() conversions will be more prevalent than the fs/dax.c radix usages. -- 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>