Re: [PATCH RFC 09/11] misc/sgi-gru: use ptep_get() for page-table reads

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/29/26 14:36, Pedro Falcato wrote:
> On Mon, Jul 27, 2026 at 05:47:00PM +0100, Muhammad Usama Anjum wrote:
>> A leaf PMD is being read through ptep_get() by treating the PMD address
>> as PTE-sized table storage. ptep_get() now accepts hw_pte_t *, so update
>> the cast accordingly.
>>
>> pte_offset_kernel() also returns hw_pte_t *. Get pte_t value by calling
>> ptep_get().
>>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
>> ---
>>  drivers/misc/sgi-gru/grufault.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/misc/sgi-gru/grufault.c b/drivers/misc/sgi-gru/grufault.c
>> index 3557d78ee47a2..ff89d34ad2aa4 100644
>> --- a/drivers/misc/sgi-gru/grufault.c
>> +++ b/drivers/misc/sgi-gru/grufault.c
>> @@ -228,10 +228,10 @@ static int atomic_pte_lookup(struct vm_area_struct *vma, unsigned long vaddr,
>>  		goto err;
>>  #ifdef CONFIG_X86_64
>>  	if (unlikely(pmd_leaf(*pmdp)))
>> -		pte = ptep_get((pte_t *)pmdp);
>> +		pte = ptep_get((hw_pte_t *)pmdp);
>>  	else
>>  #endif
>> -		pte = *pte_offset_kernel(pmdp, vaddr);
>> +		pte = ptep_get(pte_offset_kernel(pmdp, vaddr));
>>  
>>  	if (unlikely(!pte_present(pte) ||
>>  		     (write && (!pte_write(pte) || !pte_dirty(pte)))))
> 
> This code is super, super broken. Can we remove this ASAP? For starters,
> we're using is_vm_hugetlb_page() to detect page shift, the code does not
> grab refs on the pages, uses pte_offset_kernel() on user page tables,
> does not handle PUD-level hugepages, does not handle PMD-level hugepages on !x86_64,
> does not hold page table locks nor check for pte table retraction, etc
> 
> I don't think I need to go on.
> 

Heh, when Usama first showed me an early diffstat I was like "what the hell are
drivers doing with page tables here, they shouldn't be doing that".

atomic_pte_lookup documents: "Only supports Intel large pages (2MB only) on
x86_64. ZZZ - hugepage support is incomplete"

What is this crap? :)

-- 
Cheers,

David