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

From: Toshi Kani
Date: Tue Jul 15 2014 - 20:03:00 EST

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.


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.


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