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

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Date: Fri, 21 Jun 2024 11:44:12 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>, Nick Bowler <nbowler@...conx.ca>
Cc: Hailong Liu <hailong.liu@...o.com>, Nick Bowler <nbowler@...conx.ca>,
	linux-kernel@...r.kernel.org,
	Linux regressions mailing list <regressions@...ts.linux.dev>,
	linux-mm@...ck.org, sparclinux@...r.kernel.org,
	"Uladzislau Rezki (Sony)" <urezki@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: PROBLEM: kernel crashes when running xfsdump since ~6.4

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:
> > > > After upgrading my sparc to 6.9.5 I noticed that attempting to run
> > > > xfsdump instantly (within a couple seconds) and reliably crashes the
> > > > kernel.  The same problem is also observed on 6.10-rc4.
> > > [...]
> > > >   062eacf57ad91b5c272f89dc964fd6dd9715ea7d is the first bad commit
> > > >   commit 062eacf57ad91b5c272f89dc964fd6dd9715ea7d
> > > >   Author: Uladzislau Rezki (Sony) <urezki@...il.com>
> > > >   Date:   Thu Mar 30 21:06:38 2023 +0200
> > > >
> > > >       mm: vmalloc: remove a global vmap_blocks xarray
> > >
> > > I think I might see what is happening here.
> > >
> > > On this machine, there are two CPUs numbered 0 and 2 (there is no CPU1).
> > >
> > +Baoquan
> 
> Thanks for adding me, Hailong.
> 
> > 
> > Ahh, I thought you are right. addr_to_vb_xa assume that the CPU numbers are
> > contiguous. I don't have knowledge about CPU at all.
> > Technically change the implement addr_to_vb_xa() to
> > return &per_cpu(vmap_block_queue, raw_smp_processor_id()).vmap_blocks;
> > would also work, but it violate the load balance. Wating for
> > experts reply.
> 
> Yeah, I think so as you explained.
> 
> > 
> > > The per-cpu variables in mm/vmalloc.c are initialized like this, in
> > > vmalloc_init
> > >
> > >   for_each_possible_cpu(i) {
> > >     /* ... */
> > >     vbq = &per_cpu(vmap_block_queue, i);
> > >     /* initialize stuff in vbq */
> > >   }
> > >
> > > This loops over the set bits of cpu_possible_mask, bits 0 and 2 are set,
> > > so it initializes stuff with i=0 and i=2, skipping i=1 (I added prints to
> > > confirm this).
> > >
> > > Then, in vm_map_ram, with the problematic change it calls the new
> > > function addr_to_vb_xa, which does this:
> > >
> > >   int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> > >   return &per_cpu(vmap_block_queue, index).vmap_blocks;
> > >
> > > The num_possible_cpus() function counts the number of set bits in
> > > cpu_possible_mask, so it returns 2.  Thus, index is either 0 or 1, which
> > > does not correspond to what was initialized (0 or 2).  The crash occurs
> > > when the computed index is 1 in this function.  In this case, the
> > > returned value appears to be garbage (I added prints to confirm this).
> 
> This is a great catch. 
> 
Indeed :)

> > >
> > > If I change addr_to_vb_xa function to this:
> > >
> > >   int index = ((addr / VMAP_BLOCK_SIZE) & 1) << 1; /* 0 or 2 */
> > >   return &per_cpu(vmap_block_queue, index).vmap_blocks;
> 
> Yeah, while above change is not generic, e.g if it's CPU0 and CPU3.
> I think we should take the max possible CPU number as the hush bucket
> size. The vb->va is also got from global free_vmap_area, so no need to
> worry about the waste.
>
Agree.

> 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.

Or i missed something in your patch, Baoquan?

--
Uladzislau Rezki