[PATCH 1/1] sched/debug: fix dentry leak in update_sched_domain_debugfs
Kuyo Chang
kuyo.chang at mediatek.com
Fri Sep 2 03:47:18 PDT 2022
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 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.
> > >
> >
> > 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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