Re: [PATCH 1/1] kasan, vmalloc: avoid lock contention when depopulating vmalloc

[Uladzislau Rezki <urezki@gmail.com>]

Hello, Adrian!

> > >
> > > From: Adrian Huang <ahuang12@lenovo.com>
> > > After re-visiting code path about setting the kasan ptep (pte pointer),
> > > it's unlikely that a kasan ptep is set and cleared simultaneously by
> > > different CPUs. So, use ptep_get_and_clear() to get rid of the spinlock
> > > operation.
> >
> > "unlikely" isn't particularly comforting.  We'd prefer to never corrupt
> > pte's!
> >
> > I'm suspecting we need a more thorough solution here.
> >
> > btw, for a lame fix, did you try moving the spin_lock() into
> > kasan_release_vmalloc(), around the apply_to_existing_page_range()
> > call?  That would at least reduce locking frequency a lot.  Some
> > mitigation might be needed to avoid excessive hold times.
> 
> I did try it before. That didn't help. In this case, each iteration in
> kasan_release_vmalloc_node() only needs to clear one pte. However,
> vn->purge_list is the long list under the heavy load: 128 cores (128
> vmap_nodes) execute kasan_release_vmalloc_node() to clear the corresponding
> pte(s) while other cores allocate vmalloc space (populate the page table
> of the vmalloc address) and populate vmalloc shadow page table. Lots of
> cores contend init_mm.page_table_lock.
> 
> For a lame fix, adding cond_resched() in the loop of
> kasan_release_vmalloc_node() is an option.
> 
> Any suggestions and comments about this issue?
> 
One question. Do you think that running a KASAN kernel and stressing
the vmalloc allocator is an issue here? It is a debug kernel, which
implies it is slow. Also, please note, the synthetic stress test is
not a real workload, it is tighten in a hard loop to stress it as much
as we can.

Can you trigger such splat using a real workload. For example running
stress-ng --fork XXX or any different workload?

Thanks!

--
Uladzislau Rezki