It already happened... On July 15, 2014 5:28:40 PM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >On Tue, Jul 15, 2014 at 4:53 PM, Toshi Kani <toshi.kani@xxxxxx> wrote: >> On Tue, 2014-07-15 at 16:36 -0700, Andy Lutomirski wrote: >>> On Tue, Jul 15, 2014 at 4:10 PM, Toshi Kani <toshi.kani@xxxxxx> >wrote: >>> > On Tue, 2014-07-15 at 12:56 -0700, Andy Lutomirski wrote: >>> >> On Tue, Jul 15, 2014 at 12:34 PM, Toshi Kani <toshi.kani@xxxxxx> >wrote: >>> >> > This patch changes reserve_memtype() to handle the new WT type. >>> >> > When (!pat_enabled && new_type), it continues to set either WB >>> >> > or UC- to *new_type. When pat_enabled, it can reserve a given >>> >> > non-RAM range for WT. At this point, it may not reserve a RAM >>> >> > range for WT since reserve_ram_pages_type() uses the page flags >>> >> > limited to three memory types, WB, WC and UC. >>> >> >>> >> FWIW, last time I looked at this, it seemed like all the fancy >>> >> reserve_ram_pages stuff was unnecessary: shouldn't the RAM type >be >>> >> easy to track in the direct map page tables? >>> > >>> > Are you referring the direct map page tables as the kernel page >>> > directory tables (pgd/pud/..)? >>> > >>> > I think it needs to be able to keep track of the memory type per a >>> > physical memory range, not per a translation, in order to prevent >>> > aliasing of the memory type. >>> >>> Actual RAM (the lowmem kind, which is all of it on x86_64) is mapped >>> linearly somewhere in kernel address space. The pagetables for that >>> mapping could be used as the canonical source of the memory type for >>> the ram range in question. >>> >>> This only works for lowmem, so maybe it's not a good idea to rely on >it. >> >> Right. >> >> I think using struct page table for the RAM ranges is a good way for >> saving memory, but I wonder how often the RAM ranges are mapped other >> than WB... If not often, reserve_memtype() could simply call >> rbt_memtype_check_insert() for all ranges, including RAM. >> >> In this patch, I left using reserve_ram_pages_type() since I do not >see >> much reason to use WT for RAM, either. > >I hereby predict that someone, some day, will build a system with >nonvolatile "RAM", and someone will want this feature. Just saying :) > >More realistically, someone might want to write a silly driver that >lets programs mmap some WT memory for testing. > >--Andy -- Sent from my mobile phone. Please pardon brevity and lack of formatting. -- 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>