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

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Date:   Fri, 1 Jul 2022 10:42:33 +0800
From:   Haibo Li <haibo.li@...iatek.com>
To:     <samitolvanen@...gle.com>
CC:     <andrealmeid@...lia.com>, <atomlin@...hat.com>,
        <christophe.leroy@...roup.eu>, <dmitry.torokhov@...il.com>,
        <haibo.li@...iatek.com>, <jgross@...e.com>,
        <keescook@...omium.org>, <lecopzer.chen@...iatek.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <llvm@...ts.linux.dev>,
        <matthias.bgg@...il.com>, <mcgrof@...nel.org>,
        <mhiramat@...nel.org>, <nathan@...nel.org>,
        <ndesaulniers@...gle.com>, <peterz@...radead.org>,
        <xiaoming.yu@...iatek.com>, <yangtiezhu@...ngson.cn>
Subject: Re: [PATCH 2/2] ANDROID: cfi: free old cfi shadow asynchronously

> Hi,
> 
> On Thu, Jun 30, 2022 at 2:47 AM Haibo Li <haibo.li@...iatek.com> 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@...iatek.com>
> > Signed-off-by: Lecopzer Chen <lecopzer.chen@...iatek.com>
> 
> Thanks for the patch! Please drop ANDROID: from the subject line, that's only
> used in the Android kernel trees.
> 
   Thanks for the comment.I will change it in next patch.  
> >  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.
   Thanks for the comment.I will change it in next patch.
> 
> > +{
> > +       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?
> 
   Since call_rcu and rcu callbacks change members in &prev->rcu,the old pages need to be rw at first.