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

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Jun 21, 2024 at 01:42:28PM +0000, Michael Kelley wrote:
> From: Uladzislau Rezki <urezki@gmail.com> Sent: Friday, June 21, 2024 2:44 AM
> > 
> > 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:
> 
> [snip]
> 
> > > > > 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 :)
> > 
> 
> +1
> 
> More broadly, throughout kernel code there are a number of places
> that incorrectly assume the cpu_possible_mask has no gaps in it. The
> typical case does kcalloc() or kmalloc_array() with num_possible_cpus()
> as the first argument, then indexes into the allocated array with a CPU
> number from smp_processor_id() or a variant. These places should be
> using nr_cpu_ids instead of num_possible_cpus().
> 
> I'm usually working on the code for Linux guests on Hyper-V, and
> there are six occurrences in that code. While they probably don't
> have immediate practical impact because I don't think the ACPI MADT
> in a such a VM would have a gap in the processor enumeration,
> I'm planning to do fixes in the interest of general correctness.
> 
Thank you for valuable information!

--
Uladzislau Rezki