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

Yu, Yu-cheng
2020-12-08T20:32:19Z

On 12/8/2020 10:47 AM, Borislav Petkov wrote:
> On Tue, Dec 08, 2020 at 10:25:15AM -0800, Yu, Yu-cheng wrote:
>>> Both are "R/O + _PAGE_COW". Where's the difference? The dirty bit?
>>
>> The PTEs are the same for both (a) and (b), but come from different routes.
>
> Do not be afraid to go into detail and explain to me what those routes
> are please.

Case (a) is a normal writable data page that has gone through fork().
So it has W=0, D=1. But here, the software chooses not to use the D
bit, and instead, W=0, COW=1.

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.

>>>> (e) A page where the processor observed a Write=1 PTE, started a write, set
>>>> Dirty=1, but then observed a Write=0 PTE.
>>>
>>> How does that happen? Something changed the PTE's W bit to 0 in-between?
>>
>> Yes.
>
> Also do not scare from going into detail and explaining what you mean
> here. Example?

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.

>>> Does _PAGE_COW mean dirty too?
>>
>> Yes. Basically [read-only & dirty] is created by software. Now the
>> software uses a different bit.
>
> That convention:
>
> "[read-only & dirty] is created by software."
>
> needs some prominent writeup somewhere explaining what it is.
>
> Thx.
>

I will put these into the comments.

--
Yu-cheng