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

[Nick Bowler <nbowler@draconx.ca>]

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@gmail.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).

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).

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;

xfsdump is working again.

Cheers,
  Nick