Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512

From: "Christoph Lameter (Ampere)" <cl@linux.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
	 Catalin Marinas <catalin.marinas@arm.com>,
	 Sudeep Holla <sudeep.holla@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	 "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	 "Rafael J. Wysocki" <rafael@kernel.org>,
	Viresh Kumar <vireshk@kernel.org>,  Will Deacon <will@kernel.org>,
	Jonathan.Cameron@huawei.com,  Matteo.Carlini@arm.com,
	Valentin.Schneider@arm.com,  akpm@linux-foundation.org,
	anshuman.khandual@arm.com,  Eric Mackay <eric.mackay@oracle.com>,
	dave.kleikamp@oracle.com,  linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,  linux-mm@kvack.org,
	robin.murphy@arm.com,  vanshikonda@os.amperecomputing.com,
	yang@os.amperecomputing.com,  Nishanth Menon <nm@ti.com>,
	Stephen Boyd <sboyd@kernel.org>
Subject: Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512
Date: Thu, 14 Mar 2024 10:01:01 -0700 (PDT)	[thread overview]
Message-ID: <ec8f9afa-ea35-9a73-7238-b29d4a32dacb@linux.com> (raw)
In-Reply-To: <ZfL40N6HYzEQaEj1@shell.armlinux.org.uk>

On Thu, 14 Mar 2024, Russell King (Oracle) wrote:

>> It is really surprising that this didn't blow up for anyone else so
>> far... This means that the $subject patch is fine.
>
> Wow. I guess we've been lucky with that allocation hitting memory
> containing zeros. Well done at tracking it down!

It would have blown up with slub_debug because that includes poisoning the 
contents of all allocations via the slab allocator. Why did that not 
occur? We should have seen a backtrace with data in registers etc showing 
poisoning values for an unitialized object.

Note that this was indeed triggered by OFFSTACK because 
(z)alloc_cpumask_var() only generates a kmalloc allocation if that option 
is set.

The config option was never set before my patch was applied on ARM64.