[PATCH 1/1] sched/debug: fix dentry leak in update_sched_domain_debugfs

[Greg Kroah-Hartman]

On Fri, Sep 02, 2022 at 02:40:59PM +0800, Kuyo Chang wrote:
> On Fri, 2022-09-02 at 07:26 +0200, Greg Kroah-Hartman wrote:
> > On Fri, Sep 02, 2022 at 11:15:15AM +0800, Kuyo Chang wrote:
> > > From: kuyo chang <kuyo.chang at mediatek.com>
> > > 
> > > [Syndrome]
> > > Lowmemorykiller triggered while doing hotplug stress test as below
> > > cmd:
> > > echo [0/1] > /sys/devices/system/cpu/cpu${index}/online
> > > 
> > > Rootcause:
> > > Call trace of the slab owner & usage as below after hotplug stress
> > > test(4hr).
> > > There exists dentry leak at update_sched_domain_debugfs.
> > > 
> > > Total size : 322000KB
> > > <prep_new_page+44>:
> > > <get_page_from_freelist+672>:
> > > <__alloc_pages+304>:
> > > <allocate_slab+144>:
> > > <___slab_alloc+404>:
> > > <__slab_alloc+60>:
> > > <kmem_cache_alloc+1204>:
> > > <alloc_inode+100>:
> > > <new_inode+40>:
> > > <__debugfs_create_file+172>:
> > > <update_sched_domain_debugfs+824>:
> > > <partition_sched_domains_locked+1292>:
> > > <rebuild_sched_domains_locked+576>:
> > > <cpuset_hotplug_workfn+1052>:
> > > <process_one_work+584>:
> > > <worker_thread+1008>:
> > > 
> > > [Solution]
> > > Provided by Major Chen <major.chen at samsung.com> as below link.
> > > 
> https://lore.kernel.org/lkml/20220711030341epcms5p173848e98b13c09eb2fcdf2fd7287526a@epcms5p1/
> > > update_sched_domain_debugfs() uses debugfs_lookup() to find wanted
> > > dentry(which has
> > > been created by debugfs_create_dir() before), but not call dput()
> > > to return this dentry
> > > back. This result in dentry leak even debugfs_remove() is called.
> > > 
> > > [Test result]
> > > Using below commands to check inode_cache & dentry leak.
> > > cat /proc/slabinfo | grep -w inode_cache
> > > cat /proc/slabinfo | grep -w dentry
> > > 
> > > With the patch, the inode_cache & dentry stays consistent
> > > so the lowmemorykiller will not triggered anymore.
> > > 
> > > Fixes: 8a99b6833c88 ("sched: Move SCHED_DEBUG sysctl to debugfs")
> > > 
> > > Signed-off-by: Major Chen <major.chen at samsung.com>
> > > Signed-off-by: kuyo chang <kuyo.chang at mediatek.com>
> > > Tested-by: kuyo chang <kuyo.chang at mediatek.com>
> > > 
> > > ---
> > >  kernel/sched/debug.c | 7 +++++--
> > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > > index bb3d63bdf4ae..4ffea2dc01da 100644
> > > --- a/kernel/sched/debug.c
> > > +++ b/kernel/sched/debug.c
> > > @@ -412,11 +412,14 @@ void update_sched_domain_debugfs(void)
> > >  
> > >  	for_each_cpu(cpu, sd_sysctl_cpus) {
> > >  		struct sched_domain *sd;
> > > -		struct dentry *d_cpu;
> > > +		struct dentry *d_cpu, *d_lookup;
> > >  		char buf[32];
> > >  
> > >  		snprintf(buf, sizeof(buf), "cpu%d", cpu);
> > > -		debugfs_remove(debugfs_lookup(buf, sd_dentry));
> > > +		d_lookup = debugfs_lookup(buf, sd_dentry);
> > > +		debugfs_remove(d_lookup);
> > > +		if (!IS_ERR_OR_NULL(d_lookup))
> > > +			dput(d_lookup);
> > 
> > That's odd, and means that something else is removing this file right
> > after we looked it up?  Is there a missing lock here that should be
> > used
> > instead?
> > 
> > thanks,
> > 
> > greg k-h
> 
> 
> While doing cpu hotlug, the cpu_active_mask is changed, 
> so it need to update_sched_domain_debugfs.
> 
> The original design is to recreate sd_dentry, so it doing
> debugfs_remove and then debugfs_create_dir.
> However, by debugfs_lookup function usage.
> The returned dentry must be passed to dput() when it is no longer
> needed to avoid dentry leak.

Eeeek, nice find!  I've been adding this pattern:
	debugfs_remove(debugfs_lookup(...));
all over the place, and as you point out, that's wrong!

It's as if I didn't even read the documentation I wrote.

{sigh}

Ok, as this is going to be a very common pattern, how about we create:
	debugfs_lookup_and_remove()
function that does the above logic all in one place and then we don't
have to put that logic everywhere in the kernel.  My goal is for users
of debugfs to never have to worry about anything about 'struct dentry'
at all, and I really failed that goal here in a major way.

I can work on that this afternoon after I get some other things done,
unless you want to do it now?

Again, very nice find, thank you for this.

greg k-h