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

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On Wed, Jun 26, 2024 at 07:30:39PM +0800, Hailong Liu wrote:
> On Wed, 26. Jun 18:51, Baoquan He wrote:
> > On 06/26/24 at 06:03pm, Hailong Liu wrote:
> > > On Wed, 26. Jun 11:15, Uladzislau Rezki wrote:
> > > > On Wed, Jun 26, 2024 at 01:12:06PM +0800, Hailong Liu wrote:
> > > > > On Tue, 25. Jun 22:05, Uladzislau Rezki wrote:
> > > > > > > > > > > /**
> > > > > > > > > > >  * 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.
> G> > > > > >
> > > > > > > > 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;
> > > > > IIUC, use nr_cpu_ids here maybe incorrect.
> > > > >
> > > > > take b101 as example, nr_cpu_ids is 3. if index is 2 cpumask_nth(2, cpu_possible_mask);
> > > > > might return 64.
> > > > >
> > > > But then a CPU2 becomes possible? Cutting by % nr_cpu_ids generates values < nr_cpu_ids.
> > > > So, last CPU is always possible and we never do cpumask_nth() on a last possible CPU.
> > > >
> > > > What i miss here?
> > > >
> > > Sorry, I forget to reply to all :), I write a demo to test as follows:
> > >
> > > static int cpumask_init(void)
> > > {
> > >        struct cpumask mask;
> > >        unsigned int cpu_id;
> > >        cpumask_clear(&mask);
> > >
> > >        cpumask_set_cpu(1, &mask);
> > >        cpumask_set_cpu(3, &mask);
> > >        cpumask_set_cpu(5, &mask);
> > >
> > >        cpu_id = find_last_bit(cpumask_bits(&mask), NR_CPUS) + 1;
> > >        pr_info("cpu_id:%d\n", cpu_id);
> > >
> > >        for (; i < nr_cpu_ids; i++) {
> > >                pr_info("%d: cpu_%d\n", i, cpumask_nth(i, &mask));
> > >        }
> > >
> > >        return 0;
> > > }
> > >
> > > [    1.337020][    T1] cpu_id:6
> > > [    1.337338][    T1] 0: cpu_1
> > > [    1.337558][    T1] 1: cpu_3
> > > [    1.337751][    T1] 2: cpu_5
> > > [    1.337960][    T1] 3: cpu_64
> > > [    1.338183][    T1] 4: cpu_64
> > > [    1.338387][    T1] 5: cpu_64
> > > [    1.338594][    T1] 6: cpu_64
> > >
> > > In summary, the nr_cpu_ids = last_bit + 1, and cpumask_nth() return the nth cpu_id.
> >
> > I think just using below change for a quick fix is enough. It doesn't
> > have the issue cpumask_nth() has and very simple. For most of systems,
> > it only adds an extra cpu_possible(idex) checking.
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 633363997dec..59a8951cc6c0 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(idex))
> > +		index = cpumask_next(index, cpu_possible_mask);
> >
> >  	return &per_cpu(vmap_block_queue, index).vmap_blocks;
> >  }
> >
> Agreed! This is a very simple solution.
> 
> If cpumask is b1000001, addresses being distributed across different
> CPUs could theoretically lead to such a situation, but it has not been
> encountered in practice. I’m just pointing out the possibility here.
> 
>   CPU_0  CPU_6  CPU_6  CPU_6  CPU_6  CPU_6
>     |      |      |      |      |      |
>     V      V      V      V      V      V
> 0     10     20     30     40     50     60
> |------|------|------|------|------|------|..
> 
Right!

>
> Thanks again for your reply, I learned a lot.
> 
Thank you for helping :)

--
Uladzislau Rezki




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