Re: [PATCH 1/2] mm/debug_vm_pgtable/basic: Add validation for dirtiness after write protect

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 11/27/20 3:14 PM, Catalin Marinas wrote:
> On Fri, Nov 27, 2020 at 09:22:24AM +0100, Christophe Leroy wrote:
>> Le 27/11/2020 à 06:06, Anshuman Khandual a écrit :
>>> This adds validation tests for dirtiness after write protect conversion for
>>> each page table level. This is important for platforms such as arm64 that
>>> removes the hardware dirty bit while making it an write protected one. This
>>> also fixes pxx_wrprotect() related typos in the documentation file.
>>
>>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
>>> index c05d9dcf7891..a5be11210597 100644
>>> --- a/mm/debug_vm_pgtable.c
>>> +++ b/mm/debug_vm_pgtable.c
>>> @@ -70,6 +70,7 @@ static void __init pte_basic_tests(unsigned long pfn, pgprot_t prot)
>>>   	WARN_ON(pte_young(pte_mkold(pte_mkyoung(pte))));
>>>   	WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte))));
>>>   	WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte))));
>>> +	WARN_ON(pte_dirty(pte_wrprotect(pte)));
>>
>> Wondering what you are testing here exactly.
>>
>> Do you expect that if PTE has the dirty bit, it gets cleared by pte_wrprotect() ?
>>
>> Powerpc doesn't do that, it only clears the RW bit but the dirty bit remains
>> if it is set, until you call pte_mkclean() explicitely.
> 
> Arm64 has an unusual way of setting a hardware dirty "bit", it actually
> clears the PTE_RDONLY bit. The pte_wrprotect() sets the PTE_RDONLY bit
> back and we can lose the dirty information. Will found this and posted
> patches to fix the arm64 pte_wprotect() to set a software PTE_DIRTY if
> !PTE_RDONLY (we do this for ptep_set_wrprotect() already). My concern
> was that we may inadvertently make a fresh/clean pte dirty with such
> change, hence the suggestion for the test.
> 
> That said, I think we also need a test in the other direction,
> pte_wrprotect() should preserve any dirty information:
> 
> 	WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte))));

This seems like a generic enough principle which all platforms should
adhere to. But the proposed test WARN_ON(pte_dirty(pte_wrprotect(pte)))
might fail on some platforms if the page table entry came in as a dirty
one and pte_wrprotect() is not expected to alter the dirty state.

Instead, should we just add the following two tests, which would ensure
that pte_wrprotect() never alters the dirty state of a page table entry.

WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte))));
WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(pte))));

> 
> If pte_mkwrite() makes a pte truly writable and potentially dirty, we
> could also add a test as below. However, I think that's valid for arm64,
> other architectures with a separate hardware dirty bit would fail this:
> 
> 	WARN_ON(!pte_dirty(pte_wrprotect(pte_mkwrite(pte))));

Right.