Re: [PATCH 1/1] mm: vmalloc: Remove a global vmap_blocks xarray

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Mar 23, 2023 at 02:12:53PM -0700, Andrew Morton wrote:
> On Thu, 23 Mar 2023 20:21:11 +0100 "Uladzislau Rezki (Sony)" <urezki@gmail.com> wrote:
> 
> > A global vmap_blocks-xarray array can be contented under
> > heavy usage of the vm_map_ram()/vm_unmap_ram() APIs. The
> > lock_stat shows that a "vmap_blocks.xa_lock" lock is a
> > second in a top-list when it comes to contentions:
> > 
> > ...
> > 
> > This patch does not fix vmap_area_lock/free_vmap_area_lock and
> > purge_vmap_area_lock bottle-necks, it is rather a separate rework.
> > 
> > ...
> >
> >  static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue);
> >  
> > ...
> >
> > +static struct vmap_block_queue *
> > +addr_to_vbq(unsigned long addr)
> > +{
> > +	int cpu = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus();
> > +	return &per_cpu(vmap_block_queue, cpu);
> > +}
> 
> Seems strange.  vmap_block_queue is not a per-cpu thing in this usage. 
> Instead it's a hash table, indexed off the (hashed) address, not off
> smp_processor_id().
> 
> Yet in other places, vmap_block_queue *is* used in the conventional
> cpu-local fashion.
> 
> So we can have CPU A using the cpu-local entry in vmap_block_queue
> while CPU B is simultaneously using it, having looked it up via `addr'.
> 
> AFAICT this all works OK, no races.
> 
> But still, what it's doing is mixing an addr-indexed hashtable with the
> CPU-indexed array in surprising ways.  It would be clearer to make the
> vmap_blocks array a separate thing from the per-cpu array, although it
> would presumably use a bit more memory.
> 
> Can we please at least get a big fat comment in an appropriate place
> which explains all this to the reader?
>
Yep, i will send out a v2 with all explanation. Indeed i have to add
detailed explanation.

Thanks!

--
Uladzislau Rezki