Re: [PATCH v2 16/17] xen/riscv: implement mfn_valid() and page reference, ownership handling helpers

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.07.2025 16:37, Oleksii Kurochko wrote:
> 
> On 7/2/25 12:28 PM, Jan Beulich wrote:
>> On 02.07.2025 12:09, Jan Beulich wrote:
>>> On 10.06.2025 15:05, Oleksii Kurochko wrote:
>>>> @@ -613,3 +612,91 @@ void __iomem *ioremap(paddr_t pa, size_t len)
>>>>   {
>>>>       return ioremap_attr(pa, len, PAGE_HYPERVISOR_NOCACHE);
>>>>   }
>>>> +
>>>> +int page_is_ram_type(unsigned long mfn, unsigned long mem_type)
>>>> +{
>>>> +    ASSERT_UNREACHABLE();
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +
>>>> +static struct domain *page_get_owner_and_nr_reference(struct page_info 
>>>> *page,
>>>> +                                                      unsigned long nr)
>>>> +{
>>>> +    unsigned long x, y = page->count_info;
>>>> +    struct domain *owner;
>>>> +
>>>> +    /* Restrict nr to avoid "double" overflow */
>>>> +    if ( nr >= PGC_count_mask )
>>>> +    {
>>>> +        ASSERT_UNREACHABLE();
>>>> +        return NULL;
>>>> +    }
>>> I question the validity of this, already in the Arm original: I can't spot
>>> how the caller guarantees to stay below that limit. Without such an
>>> (attempted) guarantee, ASSERT_UNREACHABLE() is wrong to use. All I can see
>>> is process_shm_node() incrementing shmem_extra[].nr_shm_borrowers, without
>>> any limit check.
>>>
>>>> +    do {
>>>> +        x = y;
>>>> +        /*
>>>> +         * Count ==  0: Page is not allocated, so we cannot take a 
>>>> reference.
>>>> +         * Count == -1: Reference count would wrap, which is invalid.
>>>> +         */
>>> May I once again ask that you look carefully at comments (as much as at 
>>> code)
>>> you copy. Clearly this comment wasn't properly updated when the bumping by 1
>>> was changed to bumping by nr.
>>>
>>>> +        if ( unlikely(((x + nr) & PGC_count_mask) <= nr) )
>>>> +            return NULL;
>>>> +    }
>>>> +    while ( (y = cmpxchg(&page->count_info, x, x + nr)) != x );
>>>> +
>>>> +    owner = page_get_owner(page);
>>>> +    ASSERT(owner);
>>>> +
>>>> +    return owner;
>>>> +}
>> There also looks to be a dead code concern here (towards the "nr" parameters
>> here and elsewhere, when STATIC_SHM=n). Just that apparently we decided to
>> leave out Misra rule 2.2 entirely.
> 
> I think that I didn't get what is an issue when STATIC_SHM=n, functions is 
> still
> going to be called through page_get_owner_and_reference(), at least, in 
> page_alloc.c .

Yes, but will "nr" ever be anything other than 1 then? IOW omitting the 
parameter
would be fine. And that's what "dead code" is about.

Jan