Re: [PATCH v2 06/14] mm: handle_pte_fault() use pte_offset_map_rw_nolock()

[David Hildenbrand <david@redhat.com>]

On 22.08.24 09:13, Qi Zheng wrote:
> In handle_pte_fault(), we may modify the vmf->pte after acquiring the
> vmf->ptl, so convert it to using pte_offset_map_rw_nolock(). But since we
> will do the pte_same() check, so there is no need to get pmdval to do
> pmd_same() check, just pass a dummy variable to it.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
>   mm/memory.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 93c0c25433d02..7b6071a0e21e2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5499,14 +5499,22 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>   		vmf->pte = NULL;
>   		vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
>   	} else {
> +		pmd_t dummy_pmdval;
> +
>   		/*
>   		 * A regular pmd is established and it can't morph into a huge
>   		 * pmd by anon khugepaged, since that takes mmap_lock in write
>   		 * mode; but shmem or file collapse to THP could still morph
>   		 * it into a huge pmd: just retry later if so.
> +		 *
> +		 * Use the maywrite version to indicate that vmf->pte will be
> +		 * modified, but since we will use pte_same() to detect the
> +		 * change of the pte entry, there is no need to get pmdval, so
> +		 * just pass a dummy variable to it.
>   		 */
> -		vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
> -						 vmf->address, &vmf->ptl);
> +		vmf->pte = pte_offset_map_rw_nolock(vmf->vma->vm_mm, vmf->pmd,
> +						    vmf->address, &dummy_pmdval,
> +						    &vmf->ptl);
>   		if (unlikely(!vmf->pte))
>   			return 0;
>   		vmf->orig_pte = ptep_get_lockless(vmf->pte);

No I understand why we don't need the PMD val in these cases ... the PTE 
would also be pte_none() at the point the page table is freed, so we 
would detect the change as well.

I do enjoy documenting why we use a dummy value, though. Likely without 
that, new users will just pass NULL and call it a day.

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

-- 
Cheers,

David / dhildenb