Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW
From: Dave Hansen <dave.hansen@linux.intel.com>
Date: 2018-09-14 21:33:43
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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?