Re: [Bug 216489] New: Machine freezes due to memory lock

From: Matthew Wilcox <willy@infradead.org>
To: Yu Zhao <yuzhao@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	dev@der-flo.net, Linux-MM <linux-mm@kvack.org>,
	Uladzislau Rezki <urezki@gmail.com>,
	bugzilla-daemon@kernel.org, Kees Cook <keescook@chromium.org>
Subject: Re: [Bug 216489] New: Machine freezes due to memory lock
Date: Fri, 16 Sep 2022 09:38:33 +0100	[thread overview]
Message-ID: <YyQ2CSdIJdvQPSPO@casper.infradead.org> (raw)
In-Reply-To: <CAOUHufZMpiuzWH7EqqDkr07iP+f80w99fQVRgiuZfc6jtsUJzA@mail.gmail.com>

On Thu, Sep 15, 2022 at 05:59:56PM -0600, Yu Zhao wrote:
> I think this is a manifest of the lockdep warning I reported a couple
> of weeks ago:
> https://lore.kernel.org/r/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@mail.gmail.com/

That would certainly match the symptoms.

Turning vmap_lock into an NMI-safe lock would be bad.  I don't even know
if we have primitives for that (it's not like you can disable an NMI
...)

I don't quite have time to write a patch right now.  Perhaps something
like:

struct vmap_area *find_vmap_area_nmi(unsigned long addr)
{
        struct vmap_area *va;

        if (spin_trylock(&vmap_area_lock))
		return NULL;
        va = __find_vmap_area(addr, &vmap_area_root);
        spin_unlock(&vmap_area_lock);

        return va;
}

and then call find_vmap_area_nmi() in check_heap_object().  I may have
the polarity of the return value of spin_trylock() incorrect.