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

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Date: Fri, 21 Jun 2024 11:30:05 +0800
From: Hailong Liu <hailong.liu@...o.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>, Baoquan He <bhe@...hat.com>, "Uladzislau Rezki
 (Sony)" <urezki@...il.com>, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: PROBLEM: kernel crashes when running xfsdump since ~6.4

On Thu, 20. Jun 14:02, Nick Bowler wrote:
> 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).
>
+Baoquan

Ahh, I thought you are right. addr_to_vb_xa assume that the CPU numbers are
contiguous. I don't have knowledge about CPU at all.
Technically change the implement addr_to_vb_xa() to
return &per_cpu(vmap_block_queue, raw_smp_processor_id()).vmap_blocks;
would also work, but it violate the load balance. Wating for
experts reply.

> 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
>
>

--
help you, help me,
Hailong.