Messages in this thread | | | Subject | 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, 14 Sep 2018 14:08:57 -0700 |
| |
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. So can we say this is probably fine in terms of > > efficiency? > Well, the first fork() will do all the hard work. I 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()? Otherwise it will not go through ptep_set_wrprotect()?
| |