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

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Date:   Wed, 20 Jul 2022 20:11:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>
Subject: Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on
 writeprotect

On 20.07.22 20:09, Nadav Amit wrote:
> On Jul 20, 2022, at 11:00 AM, David Hildenbrand <david@...hat.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.

The I'm confused how can_change_pte_writable() would ever allow for
that. Best to show me the code :)

-- 
Thanks,

David / dhildenb