Re: [PATCH v2 02/14] arm: adjust_pte() use pte_offset_map_rw_nolock()

[David Hildenbrand <david@redhat.com>]

On 22.08.24 09:13, Qi Zheng wrote:
> In do_adjust_pte(), we may modify the pte entry. At this time, the write
> lock of mmap_lock is not held, and the pte_same() check is not performed
> after the PTL held. The corresponding pmd entry may have been modified
> concurrently. Therefore, in order to ensure the stability if pmd entry,
> use pte_offset_map_rw_nolock() to replace pte_offset_map_nolock(), and do
> pmd_same() check after holding the PTL.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>   arch/arm/mm/fault-armv.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/mm/fault-armv.c b/arch/arm/mm/fault-armv.c
> index 831793cd6ff94..de6c7d8a2ddfc 100644
> --- a/arch/arm/mm/fault-armv.c
> +++ b/arch/arm/mm/fault-armv.c
> @@ -94,6 +94,7 @@ static int adjust_pte(struct vm_area_struct *vma, unsigned long address,
>   	pud_t *pud;
>   	pmd_t *pmd;
>   	pte_t *pte;
> +	pmd_t pmdval;
>   	int ret;
>   
>   	pgd = pgd_offset(vma->vm_mm, address);
> @@ -112,16 +113,22 @@ static int adjust_pte(struct vm_area_struct *vma, unsigned long address,
>   	if (pmd_none_or_clear_bad(pmd))
>   		return 0;
>   
> +again:
>   	/*
>   	 * This is called while another page table is mapped, so we
>   	 * must use the nested version.  This also means we need to
>   	 * open-code the spin-locking.
>   	 */
> -	pte = pte_offset_map_nolock(vma->vm_mm, pmd, address, &ptl);
> +	pte = pte_offset_map_rw_nolock(vma->vm_mm, pmd, address, &pmdval, &ptl);
>   	if (!pte)
>   		return 0;
>   
>   	do_pte_lock(ptl);
> +	if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
> +		do_pte_unlock(ptl);
> +		pte_unmap(pte);
> +		goto again;
> +	}
>   
>   	ret = do_adjust_pte(vma, address, pfn, pte);
>   

Looks correct to me, but I wonder why the missing pmd_same check is not 
an issue so far ... any experts? THP on __LINUX_ARM_ARCH__ < 6 is not 
really used/possible?

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb