Re: PROBLEM: kernel crashes when running xfsdump since ~6.4

[Baoquan He <bhe@redhat.com>]

On 06/25/24 at 12:32pm, Uladzislau Rezki wrote:
> On Tue, Jun 25, 2024 at 11:30:33AM +0800, Baoquan He wrote:
> > On 06/24/24 at 02:16pm, Uladzislau Rezki wrote:
> > > On Fri, Jun 21, 2024 at 10:02:50PM +0800, Baoquan He wrote:
> > > > On 06/21/24 at 11:44am, Uladzislau Rezki wrote:
> > > > > On Fri, Jun 21, 2024 at 03:07:16PM +0800, Baoquan He wrote:
> > > > > > On 06/21/24 at 11:30am, Hailong Liu wrote:
> > > > > > > On Thu, 20. Jun 14:02, Nick Bowler wrote:
> > > > > > > > On 2024-06-20 02:19, Nick Bowler wrote:
> > > > ......
> > > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > > > index be2dd281ea76..18e87cafbaf2 100644
> > > > > > --- a/mm/vmalloc.c
> > > > > > +++ b/mm/vmalloc.c
> > > > > > @@ -2542,7 +2542,7 @@ static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
> > > > > >  static struct xarray *
> > > > > >  addr_to_vb_xa(unsigned long addr)
> > > > > >  {
> > > > > > -	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> > > > > > +	int index = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids;
> > > > > >  
> > > > > >  	return &per_cpu(vmap_block_queue, index).vmap_blocks;
> > > > > >  }
> > > > > > 
> > > > > The problem i see is about not-initializing of the:
> > > > > <snip>
> > > > > 	for_each_possible_cpu(i) {
> > > > > 		struct vmap_block_queue *vbq;
> > > > > 		struct vfree_deferred *p;
> > > > > 
> > > > > 		vbq = &per_cpu(vmap_block_queue, i);
> > > > > 		spin_lock_init(&vbq->lock);
> > > > > 		INIT_LIST_HEAD(&vbq->free);
> > > > > 		p = &per_cpu(vfree_deferred, i);
> > > > > 		init_llist_head(&p->list);
> > > > > 		INIT_WORK(&p->wq, delayed_vfree_work);
> > > > > 		xa_init(&vbq->vmap_blocks);
> > > > > 	}
> > > > > <snip>
> > > > > 
> > > > > correctly or fully. It is my bad i did not think that CPUs in a possible mask
> > > > > can be non sequential :-/
> > > > > 
> > > > > nr_cpu_ids - is not the max possible CPU. For example, in Nick case,
> > > > > when he has two CPUs, num_possible_cpus() and nr_cpu_ids are the same.
> > > > 
> > > > I checked the generic version of setup_nr_cpu_ids(), from codes, they
> > > > are different with my understanding.
> > > > 
> > > > kernel/smp.c
> > > > void __init setup_nr_cpu_ids(void)
> > > > {
> > > >         set_nr_cpu_ids(find_last_bit(cpumask_bits(cpu_possible_mask), NR_CPUS) + 1);
> > > > }
> > > > 
> > > I see that it is not a weak function, so it is generic, thus the
> > > behavior can not be overwritten, which is great. This does what we
> > > need.
> > > 
> > > Thank you for checking this you are right!
> > 
> > Thanks for confirming this.
> > 
> > > 
> > > Then it is just a matter of proper initialization of the hash:
> > > 
> > > <snip>
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 5d3aa2dc88a8..1733946f7a12 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -5087,7 +5087,13 @@ void __init vmalloc_init(void)
> > >          */
> > >         vmap_area_cachep = KMEM_CACHE(vmap_area, SLAB_PANIC);
> > >  
> > > -       for_each_possible_cpu(i) {
> > > +       /*
> > > +        * We use "nr_cpu_ids" here because some architectures
> > > +        * may have "gaps" in cpu-possible-mask. It is OK for
> > > +        * per-cpu approaches but is not OK for cases where it
> > > +        * can be used as hashes also.
> > > +        */
> > > +       for (i = 0; i < nr_cpu_ids; i++) {
> > 
> > I was wrong about earlier comments. Percpu variables are only available
> > on possible CPUs. For those nonexistent possible CPUs of static percpu
> > variable vmap_block_queue, there isn't memory allocated and mapped for
> > them. So accessing into them will cause problem.
> > 
> > In Nick's case, there are only CPU0, CPU2. If you access
> > &per_cpu(vmap_block_queue, 1), problem occurs. So I think we may need to
> > change to take other way for vbq. E.g:
> > 1) Storing the vb in the nearest neighbouring vbq on possible CPU as
> >    below draft patch;
> > 2) create an normal array to store vbq of size nr_cpu_ids, then we can
> >    store/fetch each vbq on non-possible CPU?
> > 
> A correct way, i think, is to create a normal array. A quick fix can be
> to stick to a next possible CPU.
> 
> > The way 1) is simpler, the existing code can be adapted a little just as
> > below.
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 633363997dec..59a8951cc6c0 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2542,7 +2542,10 @@ static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
> >  static struct xarray *
> >  addr_to_vb_xa(unsigned long addr)
> >  {
> > -	int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> > +	int index = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids;
> > +
> > +	if (!cpu_possible(idex))
> > +		index = cpumask_next(index, cpu_possible_mask);
> >
> cpumask_next() can return nr_cpu_ids if no next bits set.

It won't. nr_cpu_ids is the largest index + 1, the hashed index will
be:  0 =<  index  <= (nr_cpu_ids - 1) e.g cpu_possible_mask is
b10001111, the nr_cpu_ids is 8, the largest bit is cpu7.
cpu_possible(index) will check that. So the largest bit of cpumask_next()
returns is (nr_cpu_ids - 1).