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

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Date: Mon, 24 Jun 2024 14:20:42 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Nick Bowler <nbowler@...conx.ca>
Cc: 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 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).
> 
> 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.
> 
Could you please test below?

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 5d3aa2dc88a8..1733946f7a12 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -5087,7 +5087,13 @@ void __init vmalloc_init(void)
         */
        vmap_area_cachep = KMEM_CACHE(vmap_area, SLAB_PANIC);

-       for_each_possible_cpu(i) {
+       /*
+        * We use "nr_cpu_ids" here because some architectures
+        * may have "gaps" in cpu-possible-mask. It is OK for
+        * per-cpu approaches but is not OK for cases where it
+        * can be used as hashes also.
+        */
+       for (i = 0; i < nr_cpu_ids; i++) {
                struct vmap_block_queue *vbq;
                struct vfree_deferred *p;
<snip>

Thank you in advance and i really appreciate for finding this
issue!

--
Uladzislau Rezki