Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Uladzislau Rezki]

On Wed, Dec 09, 2020 at 11:42:39AM -0800, Paul E. McKenney wrote:
> On Wed, Dec 09, 2020 at 08:36:37PM +0100, Uladzislau Rezki wrote:
> > On Tue, Dec 08, 2020 at 05:13:01PM -0800, paulmck@xxxxxxxxxx wrote:
> > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > > 
> > > This commit adds vmalloc() support to mem_dump_obj().  Note that the
> > > vmalloc_dump_obj() function combines the checking and dumping, in
> > > contrast with the split between kmem_valid_obj() and kmem_dump_obj().
> > > The reason for the difference is that the checking in the vmalloc()
> > > case involves acquiring a global lock, and redundant acquisitions of
> > > global locks should be avoided, even on not-so-fast paths.
> > > 
> > > Note that this change causes on-stack variables to be reported as
> > > vmalloc() storage from kernel_clone() or similar, depending on the degree
> > > of inlining that your compiler does.  This is likely more helpful than
> > > the earlier "non-paged (local) memory".
> > > 
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > Cc: <linux-mm@xxxxxxxxx>
> > > Reported-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > ---
> > >  include/linux/vmalloc.h |  6 ++++++
> > >  mm/util.c               | 12 +++++++-----
> > >  mm/vmalloc.c            | 12 ++++++++++++
> > >  3 files changed, 25 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > > index 938eaf9..c89c2be 100644
> > > --- a/include/linux/vmalloc.h
> > > +++ b/include/linux/vmalloc.h
> > > @@ -248,4 +248,10 @@ pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> > >  int register_vmap_purge_notifier(struct notifier_block *nb);
> > >  int unregister_vmap_purge_notifier(struct notifier_block *nb);
> > >  
> > > +#ifdef CONFIG_MMU
> > > +bool vmalloc_dump_obj(void *object);
> > > +#else
> > > +static inline bool vmalloc_dump_obj(void *object) { return false; }
> > > +#endif
> > > +
> > >  #endif /* _LINUX_VMALLOC_H */
> > > diff --git a/mm/util.c b/mm/util.c
> > > index 8c2449f..ee99a0a 100644
> > > --- a/mm/util.c
> > > +++ b/mm/util.c
> > > @@ -984,6 +984,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
> > >   */
> > >  void mem_dump_obj(void *object)
> > >  {
> > > +	if (kmem_valid_obj(object)) {
> > > +		kmem_dump_obj(object);
> > > +		return;
> > > +	}
> > > +	if (vmalloc_dump_obj(object))
> > > +		return;
> > >  	if (!virt_addr_valid(object)) {
> > >  		if (object == NULL)
> > >  			pr_cont(" NULL pointer.\n");
> > > @@ -993,10 +999,6 @@ void mem_dump_obj(void *object)
> > >  			pr_cont(" non-paged (local) memory.\n");
> > >  		return;
> > >  	}
> > > -	if (kmem_valid_obj(object)) {
> > > -		kmem_dump_obj(object);
> > > -		return;
> > > -	}
> > > -	pr_cont(" non-slab memory.\n");
> > > +	pr_cont(" non-slab/vmalloc memory.\n");
> > >  }
> > >  EXPORT_SYMBOL_GPL(mem_dump_obj);
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 6ae491a..7421719 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> > >  }
> > >  #endif	/* CONFIG_SMP */
> > >  
> > > +bool vmalloc_dump_obj(void *object)
> > > +{
> > > +	struct vm_struct *vm;
> > > +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > >
> > Paul, vmalloced addresses are already aligned to PAGE_SIZE, so that one
> > is odd.
> 
> They are, but this is to handle things like this:
> 
> 	struct foo {
> 		int a;
> 		struct rcu_head rh;
> 	};
> 
> 	void silly(struct foo *fp)
> 	{
> 		call_rcu(&fp->rh, my_rcu_cb);
> 		call_rcu(&fp->rh, my_other_rcu_cb);
> 	}
> 
> In kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, this would
> result in a call to mem_dump_obj() and then to vmalloc_dump_obj()
> with a non-page-aligned pointer.
> 
OK, i got it. I thought the functions deals with original vmalloc
pointer. In fact it is not :)

--
Vlad Rezki