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

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Message-Id: <20240925134706.2a0c2717a41a338d938581ff@linux-foundation.org>
Date: Wed, 25 Sep 2024 13:47:06 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Adrian Huang <adrianhuang0701@...il.com>
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 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.