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

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Message-ID: <CAHKZfL0D6UXvhuiq_GQgCwdKZAQ7CEkajJPpZJ40_e+ZfvHvcw@mail.gmail.com>
Date: Thu, 26 Sep 2024 20:22:54 +0800
From: Huang Adrian <adrianhuang0701@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Uladzislau Rezki <urezki@...il.com>, kasan-dev@...glegroups.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Adrian Huang <ahuang12@...ovo.com>
Subject: Re: [PATCH 1/1] kasan, vmalloc: avoid lock contention when
 depopulating vmalloc

On Thu, Sep 26, 2024 at 4:47 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 25 Sep 2024 21:47:32 +0800 Adrian Huang <adrianhuang0701@...il.com> wrote:
>
> >
> > ...
> >
> > From: Adrian Huang <ahuang12@...ovo.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