Re: [PATCH v8 05/17] iov_iter: Introduce fault_in_iov_iter_writeable

On Tue, Oct 19, 2021 at 03:41:52PM +0200, Andreas Gruenbacher wrote:
> diff --git a/mm/gup.c b/mm/gup.c
> index a7efb027d6cf..614b8536b3b6 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1691,6 +1691,69 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
>  }
>  EXPORT_SYMBOL(fault_in_writeable);
>  
> +/*
> + * fault_in_safe_writeable - fault in an address range for writing
> + * @uaddr: start of address range
> + * @size: length of address range
> + *
> + * Faults in an address range using get_user_pages, i.e., without triggering
> + * hardware page faults.  This is primarily useful when we already know that
> + * some or all of the pages in the address range aren't in memory.
> + *
> + * Other than fault_in_writeable(), this function is non-destructive.
> + *
> + * Note that we don't pin or otherwise hold the pages referenced that we fault
> + * in.  There's no guarantee that they'll stay in memory for any duration of
> + * time.
> + *
> + * Returns the number of bytes not faulted in, like copy_to_user() and
> + * copy_from_user().
> + */
> +size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
> +{
> +	unsigned long start = (unsigned long)uaddr;
> +	unsigned long end, nstart, nend;
> +	struct mm_struct *mm = current->mm;
> +	struct vm_area_struct *vma = NULL;

For arm64 tagged addresses we need the diff below, otherwise the
subsequent find_vma() will fail:

diff --git a/mm/gup.c b/mm/gup.c
index f5f362cb4640..2c51e9748a6a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1713,7 +1713,7 @@ EXPORT_SYMBOL(fault_in_writeable);
  */
 size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
 {
-	unsigned long start = (unsigned long)uaddr;
+	unsigned long start = (unsigned long)untagged_addr(uaddr);
 	unsigned long end, nstart, nend;
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = NULL;

-- 
Catalin