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: <35EEB9F0-D99A-4664-9628-27029B52CFD1@gmail.com>
Date:   Mon, 3 Oct 2022 16:17:51 -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 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?

>> Perhaps you should at least add some
>> assertion just to ensure nothing breaks.
> 
> You mean in ptep_set_access_flags()? I think some assertions would be
> really great, I'm just not sure where.

Yes, on x86’s version of the function.