Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW

Borislav Petkov
2020-12-10T17:48:45Z

On Tue, Dec 08, 2020 at 11:24:16AM -0800, Yu, Yu-cheng wrote:
> Case (a) is a normal writable data page that has gone through fork(). So it

Writable?

> has W=0, D=1. But here, the software chooses not to use the D bit, and

But it has W=0. So not writable?

> instead, W=0, COW=1.

So the "new" way of denoting that the page is modified is COW=1
*when* on CET hw. The D=1 bit is still used on the rest thus the two
_PAGE_DIRTY_BITS.

Am I close?

> Case (b) is a normal read-only data page. Since it is read-only, fork()
> won't affect it. In __get_user_pages(), a copy of the read-only page is
> needed, and the page is duplicated. The software sets COW=1 for the new
> copy.

That makes more sense.

> Thread-A is writing to a writable page, and the page's PTE is becoming W=1,
> D=1. In the middle of it, Thread-B is changing the PTE to W=0.

Yah, add that to the explanation pls.

--
Regards/Gruss,
Boris.