linux-kernel - Re: [PATCH] powerpc/kvm: silence kmemleak false positives

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Date:   Wed, 13 May 2020 14:00:44 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Qian Cai <cai@....pw>, paulus@...abs.org, benh@...nel.crashing.org,
        kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/kvm: silence kmemleak false positives

Catalin Marinas <catalin.marinas@....com> writes:
> On Mon, May 11, 2020 at 09:15:55PM +1000, Michael Ellerman wrote:
>> Qian Cai <cai@....pw> writes:
>> > kvmppc_pmd_alloc() and kvmppc_pte_alloc() allocate some memory but then
>> > pud_populate() and pmd_populate() will use __pa() to reference the newly
>> > allocated memory. The same is in xive_native_provision_pages().
>> >
>> > Since kmemleak is unable to track the physical memory resulting in false
>> > positives, silence those by using kmemleak_ignore().
>> 
>> There is kmemleak_alloc_phys(), which according to the docs can be used
>> for tracking a phys address.
>
> This won't help. While kmemleak_alloc_phys() allows passing a physical
> address, it doesn't track physical address references to this object. It
> still expects VA pointing to it, otherwise the object would be reported
> as a leak.

OK, thanks for clarifying that.

> We currently only call this from the memblock code with a min_count of
> 0, meaning it will not be reported as a leak if no references are found.
>
> We don't have this issue with page tables on other architectures since
> most of them use whole page allocations which aren't tracked by
> kmemleak. These powerpc functions use kmem_cache_alloc() which would be
> tracked automatically by kmemleak. While we could add a phys alias to
> kmemleak (another search tree), I think the easiest is as per Qian's
> patch, just ignore those objects.

Agreed.

cheers