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

[Huang Adrian <adrianhuang0701@gmail.com>]

On Thu, Sep 26, 2024 at 4:47 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 25 Sep 2024 21:47:32 +0800 Adrian Huang <adrianhuang0701@gmail.com> wrote:
>
> >
> > ...
> >
> > 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?

Thanks.

-- Adrian