From: Uladzislau Rezki <urezki@xxxxxxxxx> 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.
Michael
