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Date: Thu, 14 Mar 2024 10:01:01 -0700 (PDT)
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, 
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    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
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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.