Re: [RFC PATCH 3/11] x86, mm, pat: Change reserve_memtype() to handle WT type

[Andy Lutomirski]

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
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