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@xxxxxxxxx>
> > > > 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
