Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on writeprotect

[Nadav Amit <namit@vmware.com>]

On Jul 20, 2022, at 11:00 AM, David Hildenbrand <david@redhat.com> wrote:

> My patch review skills have seen better days. I thought you'd be
> removing the pte_write() check ... :( Tired eyes ...
> 
>> Having said that, I do notice now that pte_mkdirty() should not be done
>> only this condition is fulfilled. Instead we should just have
>> something like:
>> 
>> if (will_need) {
>> ptent = pte_mkyoung(ptent);
>> if (pte_write(ptent))
>> ptent = pte_mkdirty(ptent);
>> }
> 
> As can_change_pte_writable() will fail if it stumbles over a !pte_dirty
> page in current code ... so I assume you would have that code before the
> actual pte_mkwrite() logic, correct?

No, I thought this should go after for 2 reasons:

1. You want to allow the PTE to be made writable following the
can_change_pte_writable().

2. You do not want to set a non-writable PTE as dirty, especially since it
might then be used to determine that the PTE can become writable. Doing so
would circumvent cases in which set_page_dirty() needs to be called and
break things down.