Re: [PATCH] gup: Avoid multiple user access locking/unlocking in fault_in_{read/write}able

[Andreas Gruenbacher]

On Tue, Nov 16, 2021 at 6:50 PM Christophe Leroy
<christophe.leroy@xxxxxxxxxx> wrote:
> fault_in_readable() and fault_in_writeable() perform __get_user()
> and __put_user() in a loop, implying multiple user access
> locking/unlocking.
>
> To avoid that, use user access blocks.
>
> Cc: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>

Reviewed-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>

> ---
>  mm/gup.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 2c51e9748a6a..be2a41feec7d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1672,21 +1672,22 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
>
>         if (unlikely(size == 0))
>                 return 0;
> +       if (!user_write_access_begin(uaddr, size))
> +               return size;
>         if (!PAGE_ALIGNED(uaddr)) {
> -               if (unlikely(__put_user(0, uaddr) != 0))
> -                       return size;
> +               unsafe_put_user(0, uaddr, out);
>                 uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr);
>         }
>         end = (char __user *)PAGE_ALIGN((unsigned long)start + size);
>         if (unlikely(end < start))
>                 end = NULL;
>         while (uaddr != end) {
> -               if (unlikely(__put_user(0, uaddr) != 0))
> -                       goto out;
> +               unsafe_put_user(0, uaddr, out);
>                 uaddr += PAGE_SIZE;
>         }
>
>  out:
> +       user_write_access_end();
>         if (size > uaddr - start)
>                 return size - (uaddr - start);
>         return 0;
> @@ -1771,21 +1772,22 @@ size_t fault_in_readable(const char __user *uaddr, size_t size)
>
>         if (unlikely(size == 0))
>                 return 0;
> +       if (!user_read_access_begin(uaddr, size))
> +               return size;
>         if (!PAGE_ALIGNED(uaddr)) {
> -               if (unlikely(__get_user(c, uaddr) != 0))
> -                       return size;
> +               unsafe_get_user(c, uaddr, out);
>                 uaddr = (const char __user *)PAGE_ALIGN((unsigned long)uaddr);
>         }
>         end = (const char __user *)PAGE_ALIGN((unsigned long)start + size);
>         if (unlikely(end < start))
>                 end = NULL;
>         while (uaddr != end) {
> -               if (unlikely(__get_user(c, uaddr) != 0))
> -                       goto out;
> +               unsafe_get_user(c, uaddr, out);
>                 uaddr += PAGE_SIZE;
>         }
>
>  out:
> +       user_read_access_end();
>         (void)c;
>         if (size > uaddr - start)
>                 return size - (uaddr - start);
> --
> 2.31.1
>