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

From: Dave Hansen
Date: Fri Sep 14 2018 - 17:33:44 EST

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

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

>> 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:
> Â Â 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?