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

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

[Kuyo Chang]

From: kuyo chang <kuyo.chang@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@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@samsung.com>
Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
Tested-by: kuyo chang <kuyo.chang@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);
 		d_cpu = debugfs_create_dir(buf, sd_dentry);
 
 		i = 0;
-- 
2.18.0


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

[Greg Kroah-Hartman]

On Fri, Sep 02, 2022 at 11:15:15AM +0800, Kuyo Chang wrote:
> From: kuyo chang <kuyo.chang@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@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@samsung.com>
> Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> Tested-by: kuyo chang <kuyo.chang@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

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

[Kuyo Chang]

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@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@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@samsung.com>
> > Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> > Tested-by: kuyo chang <kuyo.chang@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.



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Re: [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@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@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@samsung.com>
> > > Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> > > Tested-by: kuyo chang <kuyo.chang@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

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

[Kuyo Chang]

On Fri, 2022-09-02 at 08:58 +0200, Greg Kroah-Hartman wrote:
> 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@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@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@samsung.com>
> > > > Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> > > > Tested-by: kuyo chang <kuyo.chang@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.
> 

Thanks for your kindly support !
Please help to add debugfs_lookup_and_remove() and then we can use
this api to fix this denrty leak issue.


> greg k-h


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

[Greg Kroah-Hartman]

On Fri, Sep 02, 2022 at 03:36:08PM +0800, Kuyo Chang wrote:
> On Fri, 2022-09-02 at 08:58 +0200, Greg Kroah-Hartman wrote:
> > 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@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@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@samsung.com>
> > > > > Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> > > > > Tested-by: kuyo chang <kuyo.chang@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.
> > 
> 
> Thanks for your kindly support !
> Please help to add debugfs_lookup_and_remove() and then we can use
> this api to fix this denrty leak issue.

Ok, can you test the patch below with your reproducer to see if it is
fixed or not?  If this resolves the issue, I'll split it up into the
proper set of patches and sweep the whole tree as every user of
debugfs_lookup() that I've found so far got it wrong :(

thanks again!

greg k-h

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 3dcf0b8b4e93..87ccd6280a9b 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -744,6 +744,28 @@ void debugfs_remove(struct dentry *dentry)
 }
 EXPORT_SYMBOL_GPL(debugfs_remove);
 
+/**
+ * debugfs_lookup_and_remove - lookup a directory or file and recursively remove it
+ * @name: a pointer to a string containing the name of the item to look up.
+ * @parent: a pointer to the parent dentry of the item.
+ *
+ * This is the equlivant of doing something like
+ * debugfs_remove(debugfs_lookup(..)) but with the proper reference counting
+ * handled for the directory being looked up.
+ */
+void debugfs_lookup_and_remove(const char *name, struct dentry *parent)
+{
+	struct dentry *dentry;
+
+	dentry = debugfs_lookup(name, parent);
+	if (IS_ERR_OR_NULL(dentry))
+		return;
+
+	debugfs_remove(dentry);
+	dput(dentry);
+}
+EXPORT_SYMBOL_GPL(debugfs_lookup_and_remove);
+
 /**
  * debugfs_rename - rename a file/directory in the debugfs filesystem
  * @old_dir: a pointer to the parent dentry for the renamed object. This
diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index c869f1e73d75..f60674692d36 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -91,6 +91,8 @@ struct dentry *debugfs_create_automount(const char *name,
 void debugfs_remove(struct dentry *dentry);
 #define debugfs_remove_recursive debugfs_remove
 
+void debugfs_lookup_and_remove(const char *name, struct dentry *parent);
+
 const struct file_operations *debugfs_real_fops(const struct file *filp);
 
 int debugfs_file_get(struct dentry *dentry);
@@ -225,6 +227,10 @@ static inline void debugfs_remove(struct dentry *dentry)
 static inline void debugfs_remove_recursive(struct dentry *dentry)
 { }
 
+static inline void debugfs_lookup_and_remove(const char *name,
+					     struct dentry *parent)
+{ }
+
 const struct file_operations *debugfs_real_fops(const struct file *filp);
 
 static inline int debugfs_file_get(struct dentry *dentry)
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index bb3d63bdf4ae..667876da8382 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -416,7 +416,7 @@ void update_sched_domain_debugfs(void)
 		char buf[32];
 
 		snprintf(buf, sizeof(buf), "cpu%d", cpu);
-		debugfs_remove(debugfs_lookup(buf, sd_dentry));
+		debugfs_lookup_and_remove(buf, sd_dentry);
 		d_cpu = debugfs_create_dir(buf, sd_dentry);
 
 		i = 0;

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

[Kuyo Chang]

On Fri, 2022-09-02 at 11:11 +0200, Greg Kroah-Hartman wrote:
> On Fri, Sep 02, 2022 at 03:36:08PM +0800, Kuyo Chang wrote:
> > On Fri, 2022-09-02 at 08:58 +0200, Greg Kroah-Hartman wrote:
> > > 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@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@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@samsung.com>
> > > > > > Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> > > > > > Tested-by: kuyo chang <kuyo.chang@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.
> > > 
> > 
> > Thanks for your kindly support !
> > Please help to add debugfs_lookup_and_remove() and then we can use
> > this api to fix this denrty leak issue.
> 
> Ok, can you test the patch below with your reproducer to see if it is
> fixed or not?  If this resolves the issue, I'll split it up into the
> proper set of patches and sweep the whole tree as every user of
> debugfs_lookup() that I've found so far got it wrong :(
> 
> thanks again!
> 
> greg k-h
> 
> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> index 3dcf0b8b4e93..87ccd6280a9b 100644
> --- a/fs/debugfs/inode.c
> +++ b/fs/debugfs/inode.c
> @@ -744,6 +744,28 @@ void debugfs_remove(struct dentry *dentry)
>  }
>  EXPORT_SYMBOL_GPL(debugfs_remove);
>  
> +/**
> + * debugfs_lookup_and_remove - lookup a directory or file and
> recursively remove it
> + * @name: a pointer to a string containing the name of the item to
> look up.
> + * @parent: a pointer to the parent dentry of the item.
> + *
> + * This is the equlivant of doing something like
> + * debugfs_remove(debugfs_lookup(..)) but with the proper reference
> counting
> + * handled for the directory being looked up.
> + */
> +void debugfs_lookup_and_remove(const char *name, struct dentry
> *parent)
> +{
> +	struct dentry *dentry;
> +
> +	dentry = debugfs_lookup(name, parent);
> +	if (IS_ERR_OR_NULL(dentry))
> +		return;
> +
> +	debugfs_remove(dentry);
> +	dput(dentry);
> +}
> +EXPORT_SYMBOL_GPL(debugfs_lookup_and_remove);
> +
>  /**
>   * debugfs_rename - rename a file/directory in the debugfs
> filesystem
>   * @old_dir: a pointer to the parent dentry for the renamed object.
> This
> diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> index c869f1e73d75..f60674692d36 100644
> --- a/include/linux/debugfs.h
> +++ b/include/linux/debugfs.h
> @@ -91,6 +91,8 @@ struct dentry *debugfs_create_automount(const char
> *name,
>  void debugfs_remove(struct dentry *dentry);
>  #define debugfs_remove_recursive debugfs_remove
>  
> +void debugfs_lookup_and_remove(const char *name, struct dentry
> *parent);
> +
>  const struct file_operations *debugfs_real_fops(const struct file
> *filp);
>  
>  int debugfs_file_get(struct dentry *dentry);
> @@ -225,6 +227,10 @@ static inline void debugfs_remove(struct dentry
> *dentry)
>  static inline void debugfs_remove_recursive(struct dentry *dentry)
>  { }
>  
> +static inline void debugfs_lookup_and_remove(const char *name,
> +					     struct dentry *parent)
> +{ }
> +
>  const struct file_operations *debugfs_real_fops(const struct file
> *filp);
>  
>  static inline int debugfs_file_get(struct dentry *dentry)
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index bb3d63bdf4ae..667876da8382 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -416,7 +416,7 @@ void update_sched_domain_debugfs(void)
>  		char buf[32];
>  
>  		snprintf(buf, sizeof(buf), "cpu%d", cpu);
> -		debugfs_remove(debugfs_lookup(buf, sd_dentry));
> +		debugfs_lookup_and_remove(buf, sd_dentry);
>  		d_cpu = debugfs_create_dir(buf, sd_dentry);
>  
>  		i = 0;

The lowmemorykiller triggered issue while doing hotplug stress is fixed
after applying your patch.
Thanks for your kindly support !



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

[Greg Kroah-Hartman]

On Fri, Sep 02, 2022 at 06:47:18PM +0800, Kuyo Chang wrote:
> On Fri, 2022-09-02 at 11:11 +0200, Greg Kroah-Hartman wrote:
> > On Fri, Sep 02, 2022 at 03:36:08PM +0800, Kuyo Chang wrote:
> > > On Fri, 2022-09-02 at 08:58 +0200, Greg Kroah-Hartman wrote:
> > > > 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@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@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@samsung.com>
> > > > > > > Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
> > > > > > > Tested-by: kuyo chang <kuyo.chang@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.
> > > > 
> > > 
> > > Thanks for your kindly support !
> > > Please help to add debugfs_lookup_and_remove() and then we can use
> > > this api to fix this denrty leak issue.
> > 
> > Ok, can you test the patch below with your reproducer to see if it is
> > fixed or not?  If this resolves the issue, I'll split it up into the
> > proper set of patches and sweep the whole tree as every user of
> > debugfs_lookup() that I've found so far got it wrong :(
> > 
> > thanks again!
> > 
> > greg k-h
> > 
> > diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> > index 3dcf0b8b4e93..87ccd6280a9b 100644
> > --- a/fs/debugfs/inode.c
> > +++ b/fs/debugfs/inode.c
> > @@ -744,6 +744,28 @@ void debugfs_remove(struct dentry *dentry)
> >  }
> >  EXPORT_SYMBOL_GPL(debugfs_remove);
> >  
> > +/**
> > + * debugfs_lookup_and_remove - lookup a directory or file and
> > recursively remove it
> > + * @name: a pointer to a string containing the name of the item to
> > look up.
> > + * @parent: a pointer to the parent dentry of the item.
> > + *
> > + * This is the equlivant of doing something like
> > + * debugfs_remove(debugfs_lookup(..)) but with the proper reference
> > counting
> > + * handled for the directory being looked up.
> > + */
> > +void debugfs_lookup_and_remove(const char *name, struct dentry
> > *parent)
> > +{
> > +	struct dentry *dentry;
> > +
> > +	dentry = debugfs_lookup(name, parent);
> > +	if (IS_ERR_OR_NULL(dentry))
> > +		return;
> > +
> > +	debugfs_remove(dentry);
> > +	dput(dentry);
> > +}
> > +EXPORT_SYMBOL_GPL(debugfs_lookup_and_remove);
> > +
> >  /**
> >   * debugfs_rename - rename a file/directory in the debugfs
> > filesystem
> >   * @old_dir: a pointer to the parent dentry for the renamed object.
> > This
> > diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> > index c869f1e73d75..f60674692d36 100644
> > --- a/include/linux/debugfs.h
> > +++ b/include/linux/debugfs.h
> > @@ -91,6 +91,8 @@ struct dentry *debugfs_create_automount(const char
> > *name,
> >  void debugfs_remove(struct dentry *dentry);
> >  #define debugfs_remove_recursive debugfs_remove
> >  
> > +void debugfs_lookup_and_remove(const char *name, struct dentry
> > *parent);
> > +
> >  const struct file_operations *debugfs_real_fops(const struct file
> > *filp);
> >  
> >  int debugfs_file_get(struct dentry *dentry);
> > @@ -225,6 +227,10 @@ static inline void debugfs_remove(struct dentry
> > *dentry)
> >  static inline void debugfs_remove_recursive(struct dentry *dentry)
> >  { }
> >  
> > +static inline void debugfs_lookup_and_remove(const char *name,
> > +					     struct dentry *parent)
> > +{ }
> > +
> >  const struct file_operations *debugfs_real_fops(const struct file
> > *filp);
> >  
> >  static inline int debugfs_file_get(struct dentry *dentry)
> > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > index bb3d63bdf4ae..667876da8382 100644
> > --- a/kernel/sched/debug.c
> > +++ b/kernel/sched/debug.c
> > @@ -416,7 +416,7 @@ void update_sched_domain_debugfs(void)
> >  		char buf[32];
> >  
> >  		snprintf(buf, sizeof(buf), "cpu%d", cpu);
> > -		debugfs_remove(debugfs_lookup(buf, sd_dentry));
> > +		debugfs_lookup_and_remove(buf, sd_dentry);
> >  		d_cpu = debugfs_create_dir(buf, sd_dentry);
> >  
> >  		i = 0;
> 
> The lowmemorykiller triggered issue while doing hotplug stress is fixed
> after applying your patch.
> Thanks for your kindly support !

Wonderful!  I added your Tested-by: to this in a 2 part patch series
that is now at:
	https://lore.kernel.org/r/20220902123107.109274-1-gregkh@linuxfoundation.org

thanks,

greg k-h

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

[Al Viro]

On Fri, Sep 02, 2022 at 11:11:54AM +0200, Greg Kroah-Hartman wrote:

> +/**
> + * debugfs_lookup_and_remove - lookup a directory or file and recursively remove it
> + * @name: a pointer to a string containing the name of the item to look up.
> + * @parent: a pointer to the parent dentry of the item.
> + *
> + * This is the equlivant of doing something like
> + * debugfs_remove(debugfs_lookup(..)) but with the proper reference counting
> + * handled for the directory being looked up.
> + */
> +void debugfs_lookup_and_remove(const char *name, struct dentry *parent)
> +{
> +	struct dentry *dentry;
> +
> +	dentry = debugfs_lookup(name, parent);
> +	if (IS_ERR_OR_NULL(dentry))
> +		return;

	Could somebody explain how could that return ERR_PTR()?

Incidentally, IS_ERR_OR_NULL is almost always a sign of bad interface - or
that of lazy cargo-culting.  Please, don't propagate that garbage... ;-/

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

[Greg Kroah-Hartman]

On Fri, Sep 02, 2022 at 03:49:27PM +0100, Al Viro wrote:
> On Fri, Sep 02, 2022 at 11:11:54AM +0200, Greg Kroah-Hartman wrote:
> 
> > +/**
> > + * debugfs_lookup_and_remove - lookup a directory or file and recursively remove it
> > + * @name: a pointer to a string containing the name of the item to look up.
> > + * @parent: a pointer to the parent dentry of the item.
> > + *
> > + * This is the equlivant of doing something like
> > + * debugfs_remove(debugfs_lookup(..)) but with the proper reference counting
> > + * handled for the directory being looked up.
> > + */
> > +void debugfs_lookup_and_remove(const char *name, struct dentry *parent)
> > +{
> > +	struct dentry *dentry;
> > +
> > +	dentry = debugfs_lookup(name, parent);
> > +	if (IS_ERR_OR_NULL(dentry))
> > +		return;
> 
> 	Could somebody explain how could that return ERR_PTR()?

If debugfs is not enabled then debugfs_lookup() will return an
ERR_PTR().  But, doh, this _IS_ debugfs so you are right, that will
never happen.

> Incidentally, IS_ERR_OR_NULL is almost always a sign of bad interface - or
> that of lazy cargo-culting.  Please, don't propagate that garbage... ;-/

Fair enough, I'll fix that up, many thanks for noticing it.

greg k-h

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