Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH v15 08/26] x86/mm: Introduce _PAGE_COW | From | "Yu, Yu-cheng" <> | Date | Tue, 8 Dec 2020 11:24:16 -0800 |
| |
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
| ![\](/images/icornerr.gif) |