Re: [PATCH v2 6/7] mm: Batch around can_change_pte_writable()

[David Hildenbrand <david@redhat.com>]

On 06.05.25 11:16, Dev Jain wrote:
> 
> 
> On 29/04/25 2:57 pm, David Hildenbrand wrote:
>> On 29.04.25 11:19, David Hildenbrand wrote:
>>>
>>>>     #include "internal.h"
>>>> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned
>>>> long addr,
>>>> -                 pte_t pte)
>>>> +bool can_change_ptes_writable(struct vm_area_struct *vma, unsigned
>>>> long addr,
>>>> +                  pte_t pte, struct folio *folio, unsigned int nr)
>>>>     {
>>>>         struct page *page;
>>>> @@ -67,8 +67,9 @@ bool can_change_pte_writable(struct vm_area_struct
>>>> *vma, unsigned long addr,
>>>>              * write-fault handler similarly would map them writable
>>>> without
>>>>              * any additional checks while holding the PT lock.
>>>>              */
>>>> -        page = vm_normal_page(vma, addr, pte);
>>>> -        return page && PageAnon(page) && PageAnonExclusive(page);
>>>> +        if (!folio)
>>>> +            folio = vm_normal_folio(vma, addr, pte);
>>>> +        return folio_test_anon(folio) && !
>>>> folio_maybe_mapped_shared(folio);
>>>
>>> Oh no, now I spot it. That is horribly wrong.
>>>
>>> Please understand first what you are doing.
>>
>> Also, would expect that the cow.c selftest would catch that:
>>
>> "vmsplice() + unmap in child with mprotect() optimization"
>>
>> After fork() we have a R/O PTE in the parent. Our child then uses
>> vmsplice() and unmaps the R/O PTE, meaning it is only left mapped by the
>> parent.
>>
>> ret = mprotect(mem, size, PROT_READ);
>> ret |= mprotect(mem, size, PROT_READ|PROT_WRITE);
>>
>> should turn the PTE writable, although it shouldn't.
>>
>> If that test case does not detect the issue you're introducing, we
>> should look into adding a test case that detects it.
>>
> 
> Hi David, I am afraid I don't understand my mistake :( PageAnon(page)
> boils down to folio_test_anon(folio). Next we want to determine whether
> the page underlying a PTE is mapped exclusively or not.

No. :)

There is your mistake.

We need to know if this folio page is *exclusive* not if the folio is 
*mapped exclusively*.

I know, it's confusing, but that's an important distinction.

You really have to test all PAE bits. I recently sent a mail on how we 
could remove PAE and encode it in the PTE value itself (W || (!W 
+dirty)), which would mean that we could batch more easily. For the time 
being, we have to stick with what we have.

-- 
Cheers,

David / dhildenb