Re: [PATCH 2/2] ANDROID: cfi: free old cfi shadow asynchronously

[Sami Tolvanen]

Hi,

On Thu, Jun 30, 2022 at 2:47 AM Haibo Li <haibo.li@xxxxxxxxxxxx> wrote:
>
> Currenly, it uses synchronize_rcu() to wait old rcu reader to go away
> in update_shadow.In embedded platform like ARM CA7X,
> load_module blocks 40~50ms in update_shadow.
> When there are more than one hundred kernel modules,
> it blocks several seconds.
>
> To accelerate load_module,change synchronize_rcu to call_rcu.
>
> Signed-off-by: Haibo Li <haibo.li@xxxxxxxxxxxx>
> Signed-off-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>

Thanks for the patch! Please drop ANDROID: from the subject line,
that's only used in the Android kernel trees.

>  kernel/cfi.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cfi.c b/kernel/cfi.c
> index 456771c8e454..a4836d59ca27 100644
> --- a/kernel/cfi.c
> +++ b/kernel/cfi.c
> @@ -43,6 +43,8 @@ typedef u16 shadow_t;
>  struct cfi_shadow {
>         /* Page index for the beginning of the shadow */
>         unsigned long base;
> +       /* rcu to free old cfi_shadow asynchronously */
> +       struct rcu_head rcu;
>         /* An array of __cfi_check locations (as indices to the shadow) */
>         shadow_t shadow[1];
>  } __packed;
> @@ -182,6 +184,13 @@ static void remove_module_from_shadow(struct cfi_shadow *s, struct module *mod,
>         }
>  }
>
> +static void _cfi_shadow_free_rcu(struct rcu_head *rcu)

I think this can be simply renamed to free_shadow.

> +{
> +       struct cfi_shadow *old = container_of(rcu, struct cfi_shadow, rcu);
> +
> +       vfree(old);
> +}
> +
>  typedef void (*update_shadow_fn)(struct cfi_shadow *, struct module *,
>                         unsigned long min_addr, unsigned long max_addr);
>
> @@ -211,11 +220,10 @@ static void update_shadow(struct module *mod, unsigned long base_addr,
>
>         rcu_assign_pointer(cfi_shadow, next);
>         mutex_unlock(&shadow_update_lock);
> -       synchronize_rcu();
>
>         if (prev) {
>                 set_memory_rw((unsigned long)prev, SHADOW_PAGES);
> -               vfree(prev);
> +               call_rcu(&prev->rcu, _cfi_shadow_free_rcu);
>         }
>  }

It's probably better to keep the pages read-only until they're
actually released. Can you move the set_memory_rw call to the new
function?

Sami