Re: [PATCH -next, v2] sched: Use struct_size() helper in task_numa_group()

[Gustavo A. R. Silva]

On Mon, Jan 10, 2022 at 07:31:58PM -0500, Steven Rostedt wrote:
> On Mon, 10 Jan 2022 23:46:15 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Mon, Jan 10, 2022 at 09:23:54AM +0800, Xiu Jianfeng wrote:
> > > Make use of struct_size() helper instead of an open-coded calculation.
> > > There is no functional change in this patch.
> > > 
> > > Link: https://github.com/KSPP/linux/issues/160
> > > Signed-off-by: Xiu Jianfeng <xiujianfeng@xxxxxxxxxx>
> > > ---
> > >  kernel/sched/fair.c | 5 ++---
> > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 095b0aa378df..af933a7f9e5d 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -2437,9 +2437,8 @@ static void task_numa_group(struct task_struct *p, int cpupid, int flags,
> > >  	int i;
> > >  
> > >  	if (unlikely(!deref_curr_numa_group(p))) {
> > > -		unsigned int size = sizeof(struct numa_group) +
> > > -				    NR_NUMA_HINT_FAULT_STATS *
> > > -				    nr_node_ids * sizeof(unsigned long);
> > > +		unsigned int size = struct_size(grp, faults,
> > > +						NR_NUMA_HINT_FAULT_STATS * nr_node_ids);  
> > 
> > Again, why?! The old code was perfectly readable, this, not so much.
> 
> Because it is unsafe, and there is an effort to get rid of all open coded
> struct_size() code. Linus has told me to do the same with my code.
> 
>   https://lore.kernel.org/all/CAHk-=wiGWjxs7EVUpccZEi6esvjpHJdgHQ=vtUeJ5crL62hx9A@xxxxxxxxxxxxxx/
> 
> And to be honest, the new change is a lot easier to read than the original
> code.

I agree.

Also, I was taking a look at the thread above and noticed the sparse
warning doesn't go away. However, the change is correct. :)

gustavo@beefy:~/git/linux$ grep 'using sizeof on a flexible structure' next-20220110.out | grep kernel/trace/trace.c
kernel/trace/trace.c:1009:17: warning: using sizeof on a flexible structure
kernel/trace/trace.c:2660:17: warning: using sizeof on a flexible structure
kernel/trace/trace.c:2770:51: warning: using sizeof on a flexible structure
kernel/trace/trace.c:3358:16: warning: using sizeof on a flexible structure
kernel/trace/trace.c:3418:16: warning: using sizeof on a flexible structure
kernel/trace/trace.c:7082:16: warning: using sizeof on a flexible structure
kernel/trace/trace.c:7160:16: warning: using sizeof on a flexible structure
gustavo@beefy:~/git/linux$ grep -nw struct_size kernel/trace/trace.c
2770:			int max_len = PAGE_SIZE - struct_size(entry, array, 1);

Thanks
--
Gustavo

> 
> struct_size() lets you know the field "faults" and the number of elements.
> You don't need to know the size of "faults". Whereas the original code,
> how is that readable? From that code, how do you know what the
> sizeof(unsigned long) is for?
> 
> -- Steve