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

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Date: Tue, 25 Jun 2024 22:05:52 +0200
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: Nick Bowler <nbowler@...conx.ca>, Hailong Liu <hailong.liu@...o.com>,
	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.
> 
Checked. There is a difference:

1. Default

<snip>
...
+   15.95%     6.05%  [kernel]        [k] __vmap_pages_range_noflush
+   15.91%     1.74%  [kernel]        [k] addr_to_vb_xa <---------------
+   15.13%    12.05%  [kernel]        [k] vunmap_p4d_range
+   14.17%    13.38%  [kernel]        [k] __find_nth_bit <--------------
+   10.62%     0.00%  [kernel]        [k] ret_from_fork_asm
+   10.62%     0.00%  [kernel]        [k] ret_from_fork
+   10.62%     0.00%  [kernel]        [k] kthread
...
<snip>

2. Check if cpu_possible() and then fallback to cpumask_nth() if not

<snip>
...
+    6.84%     0.29%  [kernel]          [k] alloc_vmap_area
+    6.80%     6.70%  [kernel]          [k] native_queued_spin_lock_slowpath
+    4.24%     0.09%  [kernel]          [k] free_vmap_block
+    2.41%     2.38%  [kernel]          [k] addr_to_vb_xa <-----------
+    1.94%     1.91%  [kernel]          [k] xas_start
...
<snip>

It is _worth_ to check if an index is in possible mask:

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 45e1506d58c3..af20f78c2cbf 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2542,7 +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;
+
+       if (!cpu_possible(index))
+               index = cpumask_nth(index, cpu_possible_mask);

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

cpumask_nth() is not cheap. My measurements are based on a synthetic
tight test and it detects a difference. In a real workloads it should
not be visible. Having gaps is not a common case plus a "slow path"
will be mitigated by the hit against possible mask.

--
Uladzislau Rezki