Date: Fri, 21 Jun 2024 19:15:45 +0800
From: Hailong Liu <hailong.liu@...o.com>
To: Uladzislau Rezki <urezki@...il.com>, Baoquan He <bhe@...hat.com>
CC: 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

On Fri, 21. Jun 11:44, Uladzislau Rezki wrote:
> On Fri, Jun 21, 2024 at 03:07:16PM +0800, Baoquan He wrote:
> > On 06/21/24 at 11:30am, Hailong Liu wrote:
> > > 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
> >
> > Thanks for adding me, Hailong.
> >
> > >
> > > 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.
> >
> > Yeah, I think so as you explained.
> >
> > >
> > > > 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).
> >
> > This is a great catch.
> >
> Indeed :)
>
> > > >
> > > > 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;
> >
> > Yeah, while above change is not generic, e.g if it's CPU0 and CPU3.
> > I think we should take the max possible CPU number as the hush bucket
> > size. The vb->va is also got from global free_vmap_area, so no need to
> > worry about the waste.
> >
> Agree.
>
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index be2dd281ea76..18e87cafbaf2 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2542,7 +2542,7 @@ 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;
> >
> >  	return &per_cpu(vmap_block_queue, index).vmap_blocks;
> >  }
> >
> The problem i see is about not-initializing of the:
> <snip>
> 	for_each_possible_cpu(i) {
> 		struct vmap_block_queue *vbq;
> 		struct vfree_deferred *p;
>
> 		vbq = &per_cpu(vmap_block_queue, i);
> 		spin_lock_init(&vbq->lock);
> 		INIT_LIST_HEAD(&vbq->free);
> 		p = &per_cpu(vfree_deferred, i);
> 		init_llist_head(&p->list);
> 		INIT_WORK(&p->wq, delayed_vfree_work);
> 		xa_init(&vbq->vmap_blocks);
> 	}
> <snip>
>
> correctly or fully. It is my bad i did not think that CPUs in a possible mask
> can be non sequential :-/
>
> nr_cpu_ids - is not the max possible CPU. For example, in Nick case,
> when he has two CPUs, num_possible_cpus() and nr_cpu_ids are the same.
>
> Or i missed something in your patch, Baoquan?
>
> --
> Uladzislau Rezki

IMO, I thought we can fix this by following.
It doesn't initialize unused variables and utilize the percpu xarray. If I said
anything wrong, please do let me know. I can learn a lot from you all :).


diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 11fe5ea208aa..f9f981674b2d 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4480,17 +4480,21 @@ void __init vmalloc_init(void)
         */
        vmap_area_cachep = KMEM_CACHE(vmap_area, SLAB_PANIC);

-       for_each_possible_cpu(i) {
+       for (i = 0; i < nr_cpu_ids; i++) {
                struct vmap_block_queue *vbq;
                struct vfree_deferred *p;

                vbq = &per_cpu(vmap_block_queue, i);
+               xa_init(&vbq->vmap_blocks);
+
+               if (!cpu_possible(i))
+                       continue;
+
                spin_lock_init(&vbq->lock);
                INIT_LIST_HEAD(&vbq->free);
                p = &per_cpu(vfree_deferred, i);
                init_llist_head(&p->list);
                INIT_WORK(&p->wq, delayed_vfree_work);
-               xa_init(&vbq->vmap_blocks);
        }

--
help you, help me,
Hailong.

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