Re: [PATCH 2/2] x86: use pv-ops in {pte,pmd}_{set,clear}_flags()

[Linus Torvalds <torvalds@linux-foundation.org>]

[ Adding x86 maintainers - Ingo was involved earlier, make it more explicit ]

On Mon, Mar 31, 2014 at 8:41 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So on x86, the obvious model is to use another bit. We've got several.
> The _PAGE_NUMA case only matters for when _PAGE_PRESENT is clear, and
> when that bit is clear the hardware doesn't care about any of the
> other bits. Currently we use:
>
>   #define _PAGE_BIT_PROTNONE      _PAGE_BIT_GLOBAL
>   #define _PAGE_BIT_FILE          _PAGE_BIT_DIRTY
>
> which are bits 8 and 6 respectively, afaik.

Side note to the x86 guys: I think it was a mistake (long long long
ago) to define these "valid when not present" bits in terms of the
"valid when present" bits.

It causes insane situations like this:

  #if _PAGE_BIT_FILE < _PAGE_BIT_PROTNONE

which makes no sense *except* if you think that those bits can have
random odd hardware-defined values. But they really can't. They are
just random bit numbers we chose.

It has *also* caused horrible pain with the whole "soft dirty" thing,
and we have absolutely ridiculous macros in pgtable-2level.h for the
insane soft-dirty case, trying to keep the swap bits spread out "just
right" to make soft-dirty (_PAGE_BIT_HIDDEN aka bit 11) not alias with
the bits we use for swap offsets etc.

So how about we just say:

 - define the bits we use when PAGE_PRESENT==0 separately and explicitly

 - clean up the crazy soft-dirty crap, preferably by just making it
depend on a 64-bit pte (so you have to have X86_PAE enabled or be on
x86-64)

that would sound like a good cleanup, because right now it's a
complete nightmare to think about which bits are used how when P is 0.
The above insane #if being the prime example of that confusion.

             Linus

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