linux-kernel - Re: [PATCH v2 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE

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Message-Id: <FF5CBF3F-A66F-4923-BEAC-D1C95DAB740D@gmail.com>
Date:   Mon, 3 Oct 2022 16:25:27 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v2 12/39] x86/mm: Update ptep_set_wrprotect() and
 pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

On Oct 3, 2022, at 4:20 PM, Nadav Amit <nadav.amit@...il.com> wrote:

> On Oct 3, 2022, at 4:17 PM, Nadav Amit <nadav.amit@...il.com> wrote:
> 
>> On Oct 3, 2022, at 3:28 PM, Edgecombe, Rick P <rick.p.edgecombe@...el.com> wrote:
>> 
>>> On Mon, 2022-10-03 at 11:11 -0700, Nadav Amit wrote:
>>>> Did you have a look at ptep_set_access_flags() and friends and
>>>> checked they
>>>> do not need to be changed too? 
>>> 
>>> ptep_set_access_flags() doesn't actually set any additional dirty bits
>>> on x86, so I think it's ok.
>> 
>> Are you sure about that? (lost my confidence today so I am hesitant).
>> 
>> Looking on insert_pfn(), I see:
>> 
>>                       entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>>                       if (ptep_set_access_flags(vma, addr, pte, entry, 1)) ...
>> 
>> This appears to set the dirty bit while potentially leaving the write-bit
>> clear. This is the scenario you want to avoid, no?
> 
> No. I am not paying attention. Ignore.

Sorry for the spam. Just this “dirty” argument is confusing. This indeed
seems like a flow that can set the dirty bit. I think.