Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

[Dave Hansen]

On 3/24/21 3:05 AM, Thomas Hellström (Intel) wrote:
> Yes, I agree. Seems like the special (SW1) is available also for huge
> page table entries on x86 AFAICT, although just not implemented.
> Otherwise the SW bits appear completely used up.

Although the _PAGE_BIT_SOFTW* bits are used up, there's plenty of room
in the hardware PTEs.  Bits 52->58 are software-available, and we're
only using 58 at the moment.

We also have not been careful at *all* about how _PAGE_BIT_SOFTW* are
used.  It's quite possible we can encode another use even in the
existing bits.

Personally, I'd just try:

#define _PAGE_BIT_SOFTW5        57      /* available for programmer */