Re: [PATCH 1/4] sched/fair: free allocated memory on error in alloc_fair_sched_group()

[Chen Yu]

On 2023-08-02 at 15:01:05 +0800, Aaron Lu wrote:
> On Wed, Jul 19, 2023 at 10:13:55AM +0800, Aaron Lu wrote:
> > On Tue, Jul 18, 2023 at 11:13:12PM +0800, Chen Yu wrote:
> > > On 2023-07-18 at 21:41:17 +0800, Aaron Lu wrote:
> > > > There is one struct cfs_rq and one struct se on each cpu for a taskgroup
> > > > and when allocation for tg->cfs_rq[X] failed, the already allocated
> > > > tg->cfs_rq[0]..tg->cfs_rq[X-1] should be freed. The same for tg->se.
> > > > 
> > > > Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
> > > > ---
> > > >  kernel/sched/fair.c | 23 ++++++++++++++++-------
> > > >  1 file changed, 16 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index a80a73909dc2..0f913487928d 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -12443,10 +12443,10 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> > > >  
> > > >  	tg->cfs_rq = kcalloc(nr_cpu_ids, sizeof(cfs_rq), GFP_KERNEL);
> > > >  	if (!tg->cfs_rq)
> > > > -		goto err;
> > > > +		return 0;
> > > >  	tg->se = kcalloc(nr_cpu_ids, sizeof(se), GFP_KERNEL);
> > > >  	if (!tg->se)
> > > > -		goto err;
> > > > +		goto err_free_rq_pointer;
> > > >  
> > > >  	tg->shares = NICE_0_LOAD;
> > > >  
> > > > @@ -12456,12 +12456,12 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> > > >  		cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
> > > >  				      GFP_KERNEL, cpu_to_node(i));
> > > >  		if (!cfs_rq)
> > > > -			goto err;
> > > > +			goto err_free;
> > > >  
> > > >  		se = kzalloc_node(sizeof(struct sched_entity_stats),
> > > >  				  GFP_KERNEL, cpu_to_node(i));
> > > >  		if (!se)
> > > > -			goto err_free_rq;
> > > > +			goto err_free;
> > > >  
> > > >  		init_cfs_rq(cfs_rq);
> > > >  		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
> > > > @@ -12470,9 +12470,18 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> > > >  
> > > >  	return 1;
> > > >  
> > > > -err_free_rq:
> > > > -	kfree(cfs_rq);
> > > > -err:
> > > > +err_free:
> > > > +	for_each_possible_cpu(i) {
> > > > +		kfree(tg->cfs_rq[i]);
> > > > +		kfree(tg->se[i]);
> > > > +
> > > > +		if (!tg->cfs_rq[i] && !tg->se[i])
> > > > +			break;
> > > > +	}
> > > > +	kfree(tg->se);
> > > > +err_free_rq_pointer:
> > > > +	kfree(tg->cfs_rq);
> > > > +
> > > Not sure if I overlooked, if alloc_fair_sched_group() fails in sched_create_group(),
> > > would sched_free_group()->free_fair_sched_group() do the cleanup?
> > 
> > You are right, I overlooked... Thanks for pointing this out.
> 
> While preparing v2, one thing still looks strange in the existing code:
> 
> int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
> {
> 	... ...
> 
> 	for_each_possible_cpu(i) {
> 		cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
> 				      GFP_KERNEL, cpu_to_node(i));
> 		if (!cfs_rq)
> 			goto err;
> 
> 		se = kzalloc_node(sizeof(struct sched_entity_stats),
> 				  GFP_KERNEL, cpu_to_node(i));
> 		if (!se)
> 			goto err_free_rq;
> 			~~~~~~~~~~~~~~~~
> 
> Since free_fair_sched_group() will take care freeing these memories on
> error path, it looks unnecessary to do this free for a random cfs_rq
> here. Or do I miss something again...?
> 
> 		init_cfs_rq(cfs_rq);
> 		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
> 		init_entity_runnable_average(se);
> 	}
> 
> 	return 1;
> 
> err_free_rq:
> 	kfree(cfs_rq);
> err:
> 	return 0;
> }
> 
> I plan to change it to this:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a80a73909dc2..ef2618ad26eb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12461,7 +12461,7 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
>  		se = kzalloc_node(sizeof(struct sched_entity_stats),
>  				  GFP_KERNEL, cpu_to_node(i));
>  		if (!se)
> -			goto err_free_rq;
> +			goto err;
>  
>  		init_cfs_rq(cfs_rq);
>  		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
> @@ -12470,8 +12470,6 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
>  
>  	return 1;
>  
> -err_free_rq:
> -	kfree(cfs_rq);
>  err:
>  	return 0;
>  }

It seems that the err_free_rq was introduced in
Commit dfc12eb26a28 ("sched: Fix memory leak in two error corner cases")
The memory leak was detected by the static tool, while that tools is
unaware of free_fair_sched_group() which will take care of this.
Not sure if a self-maintained function is preferred, or let other function
to take care of that. For now it seems to be a duplicated free. And
alloc_rt_sched_group() has the same issue.

thanks,
Chenyu