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

[Baoquan He <bhe@redhat.com>]

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?

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);
 
 	return &per_cpu(vmap_block_queue, index).vmap_blocks;
 }
@@ -2556,9 +2559,15 @@ addr_to_vb_xa(unsigned long addr)
 
 static unsigned long addr_to_vb_idx(unsigned long addr)
 {
+	int id = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids;
+	int id_dest = id; 
+
+	if (!cpu_possible(id))
+		id_dest = cpumask_next(id, cpu_possible_mask);
+
 	addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
 	addr /= VMAP_BLOCK_SIZE;
-	return addr;
+	return addr + (id_dest - id);
 }