Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW

From: Yu-cheng Yu
Date: Fri Sep 14 2018 - 17:13:40 EST

On Fri, 2018-09-14 at 13:46 -0700, Dave Hansen wrote:
> On 09/14/2018 01:39 PM, Yu-cheng Yu wrote:
> >
> > With the updated ptep_set_wrprotect() below, I did MADV_WILLNEED to a shadow
> > stack of 8 MB, then 10,000 fork()'s, but could not prove it is more or less
> > efficient than the other. ÂSo can we say this is probably fine in terms of
> > efficiency?
> Well, the first fork() will do all the hard work.ÂÂI don't think
> subsequent fork()s will be affected.

Are you talking about a recent commit:

  1b2de5d0 mm/cow: don't bother write protecting already write-protected pages

With that, subsequent fork()s will not do all the hard work.
However, I have not done that for shadow stack PTEs (do we want to do that?).
I think the additional benefit for shadow stack is small?

> Did you do something to ensure this code was being run?
> I would guess that a loop like this:
> for (i = 0; i < 10000; i++) {
> mprotect(addr, len, PROT_READ);
> mprotect(addr, len, PROT_READ|PROT_WRITE);
> }
> might show it better.

Would mprotect() do copy_one_pte()? ÂOtherwise it will not go through