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

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Message-ID:
 <SN6PR02MB4157D2FA39A5491C915D186FD4C92@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 21 Jun 2024 13:42:28 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Uladzislau Rezki <urezki@...il.com>, 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-kernel@...r.kernel.org>, Linux
 regressions mailing list <regressions@...ts.linux.dev>, "linux-mm@...ck.org"
	<linux-mm@...ck.org>, "sparclinux@...r.kernel.org"
	<sparclinux@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>
Subject: RE: PROBLEM: kernel crashes when running xfsdump since ~6.4

From: Uladzislau Rezki <urezki@...il.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.

Michael