Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

[Borislav Petkov]

On Mon, Jan 25, 2021 at 02:18:37PM -0800, Yu, Yu-cheng wrote:
> For example, when a thread reads a W=1, D=0 PTE and before changing it to
> W=0,D=0, another thread could have written to the page and the PTE is W=1,
> D=1 now.  When try_cmpxchg() detects the difference, old_pte is read again.

None of that is mentioned in the comment above it and if anything,
*that* is what should be explained there - not some guarantee about some
processors which doesn't even apply here.

Also, add the fact that try_cmpxchg() will update old_pte with any
modified bits - D=1 for example - when it fails. As Peter just explained
to me on IRC.

Thx.

-- 
Regards/Gruss,
    Boris.

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