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

On 09/14/2018 02:08 PM, Yu-cheng Yu wrote:

On Fri, 2018-09-14 at 13:46 -0700, Dave Hansen wrote:

quoted

On 09/14/2018 01:39 PM, Yu-cheng Yu wrote:

quoted

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?

BTW, I wasn't particularly concerned about shadow stacks.  Plain old
memory is affected by this change too.  Right?

quoted

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?

You're right.  mprotect() doesn't use this path.

But, that reminds me, can you take a quick look at change_pte_range()
and double-check that it's not affected by this issue?