linux-kernel - Re: [PATCH 2/4] kmemleak: Handle percpu memory allocation

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Date:	Tue, 4 Oct 2011 10:26:24 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Tejun Heo <htejun@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Huajun Li <huajun.li.lee@...il.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH 2/4] kmemleak: Handle percpu memory allocation

On Tue, Oct 04, 2011 at 10:13:14AM +0100, Tejun Heo wrote:
> On Tue, Oct 04, 2011 at 10:04:46AM +0100, Catalin Marinas wrote:
> > > The percpu part looks fine to me but I don't know how kmemleak works
> > > to judge whether the kmemleak part is okay or not.  This just avoids
> > > false positives from slab and would still require bumping up the early
> > > log memory as # of cpus increases, right?
> > 
> > No, there is only one kmemleak call for each __percpu pointer (to the
> > specific kmemleak_*_percpu function). The kmemleak expands the percpu
> > pointer into corresponding blocks for each cpu but the early log only
> > stores a single call.
> 
> Hmmm... but the following definitely seems O(#PCPU_ALLOCS * #CPUS)?
> What am I missing?
> 
> +/*
> + * Log an early allocated block and populate the stack trace.
> + */
> +static void early_alloc_percpu(struct early_log *log)
> +{
> +       unsigned int cpu;
> +       const void __percpu *ptr = log->ptr;
> +
> +       for_each_possible_cpu(cpu) {
> +               log->ptr = per_cpu_ptr(ptr, cpu);
> +               early_alloc(log);
> +       }
> +}

Before kmemleak is initialised we still get memory allocations that
kmemleak stores in an early_log buffer (via the log_early() function
called from kmemleak_alloc_percpu). Later when kmemleak has all the data
structures in place, the kmemleak_init() function goes through the
early_log array and replays the previously recorded requests. The
early_alloc_percpu() function is used during early_log replaying and it
indeed registers every percpu memory block but the early_log is always
O(#PCPU_ALLOCS).

The reason we don't call kmemleak_alloc_percpu() directly during
replaying is that early_alloc() also copies the previously recorded
stack trace into the newly created object (otherwise all early
allocations would be shown as done by kmemleak_init).

Thanks.

-- 
Catalin
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