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

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Date: Tue, 25 Jun 2024 18:49:37 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Nick Bowler <nbowler@...conx.ca>
Cc: Uladzislau Rezki <urezki@...il.com>, Hailong Liu <hailong.liu@...o.com>,
	Nick Bowler <nbowler@...conx.ca>, linux-kernel@...r.kernel.org,
	Linux regressions mailing list <regressions@...ts.linux.dev>,
	linux-mm@...ck.org, sparclinux@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: PROBLEM: kernel crashes when running xfsdump since ~6.4

> > > > /**
> > > >  * cpumask_next - get the next cpu in a cpumask
> > > >  * @n: the cpu prior to the place to search (i.e. return will be > @n)
> > > >  * @srcp: the cpumask pointer
> > > >  *
> > > >  * Return: >= nr_cpu_ids if no further cpus set.
> > > 
> > > Ah, I got what you mean. In the vbq case, it may not have chance to get
> > > a return number as nr_cpu_ids. Becuase the hashed index limits the
> > > range to [0, nr_cpu_ids-1], and cpu_possible(index) will guarantee it
> > > won't be the highest cpu number [nr_cpu_ids-1] since CPU[nr_cpu_ids-1] must
> > > be possible CPU.
> > > 
> > > Do I miss some corner cases?
> > > 
> > Right. We guarantee that a highest CPU is available by doing: % nr_cpu_ids.
> > So we do not need to use *next_wrap() variant. You do not miss anything :)
> > 
> > Hailong Liu has proposed more simpler version:
> > 
> > <snip>
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 11fe5ea208aa..e1e63ffb9c57 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1994,8 +1994,9 @@ static struct xarray *
> >  addr_to_vb_xa(unsigned long addr)
> >  {
> >         int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> > +       int cpu = cpumask_nth(index, cpu_possible_mask);
> > 
> > -       return &per_cpu(vmap_block_queue, index).vmap_blocks;
> > +       return &per_cpu(vmap_block_queue, cpu).vmap_blocks;
> > <snip>
> > 
> > which just takes a next CPU if an index is not set in the cpu_possible_mask.
> > 
> > The only thing that can be updated in the patch is to replace num_possible_cpu()
> > by the nr_cpu_ids.
> > 
> > Any thoughts? I think we need to fix it by a minor change so it is
> > easier to back-port on stable kernels.
> 
> Yeah, sounds good since the regresson commit is merged in v6.3.
> Please feel free to post this and the hash array patch separately for
> formal reviewing.
> 
Agreed! The patch about hash array i will post later.

> By the way, when I am replying this mail, I check the cpumask_nth()
> again. I doubt it may take more checking then cpu_possible(), given most
> of systems don't have gaps in cpu_possible_mask. I could be dizzy at
> this moment.
> 
> static inline unsigned int cpumask_nth(unsigned int cpu, const struct cpumask *srcp)
> {
>         return find_nth_bit(cpumask_bits(srcp), small_cpumask_bits, cpumask_check(cpu));
> }
> 
Yep, i do not think it is a big problem based on your noted fact.

Nick, could you please test your machine with below change?

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 45e1506d58c3..5458fd2290cf 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2542,9 +2542,10 @@ static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
 static struct xarray *
 addr_to_vb_xa(unsigned long addr)
 {
-       int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
+       int index = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids;
+       int cpu = cpumask_nth(index, cpu_possible_mask);

-       return &per_cpu(vmap_block_queue, index).vmap_blocks;
+       return &per_cpu(vmap_block_queue, cpu).vmap_blocks;
 }

 /*
<snip>

Thank you in advance!

--
Uladzislau Rezki