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

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Date: Thu, 14 Mar 2024 10:01:01 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...ux.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
cc: Marek Szyprowski <m.szyprowski@...sung.com>, 
    Catalin Marinas <catalin.marinas@....com>, 
    Sudeep Holla <sudeep.holla@....com>, Mark Rutland <mark.rutland@....com>, 
    "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>, 
    "Rafael J. Wysocki" <rafael@...nel.org>, Viresh Kumar <vireshk@...nel.org>, 
    Will Deacon <will@...nel.org>, Jonathan.Cameron@...wei.com, 
    Matteo.Carlini@....com, Valentin.Schneider@....com, 
    akpm@...ux-foundation.org, anshuman.khandual@....com, 
    Eric Mackay <eric.mackay@...cle.com>, dave.kleikamp@...cle.com, 
    linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org, robin.murphy@....com, 
    vanshikonda@...amperecomputing.com, yang@...amperecomputing.com, 
    Nishanth Menon <nm@...com>, Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase
 supported CPUs to 512

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.