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

[Uladzislau Rezki <urezki@gmail.com>]

Hello, Adrian!

> Hello Uladzislau,
> 
> On Fri, Sep 27, 2024 at 12:16 AM Uladzislau Rezki <urezki@gmail.com> wrote:
> >
> > 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.
> 
> Totally agree.
> 
> > Can you trigger such splat using a real workload. For example running
> > stress-ng --fork XXX or any different workload?
> 
> No, the issue could not be reproduced with stress-ng (over-weekend stress).
> 
> So, please ignore it. Sorry for the noise.
> 
No problem. This is a regular workflow what is normal, IMO :)

--
Uladzislau Rezki