linux-kernel - Re: [PATCH 2.6.20-rc1 00/10] Kernel memory leak detector 0.13

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Message-ID: <b0943d9e0612271615r42c7f6abt38f36bbd9c94319f@mail.gmail.com>
Date:	Thu, 28 Dec 2006 00:15:41 +0000
From:	"Catalin Marinas" <catalin.marinas@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.20-rc1 00/10] Kernel memory leak detector 0.13

On 27/12/06, Ingo Molnar <mingo@...e.hu> wrote:
> * Ingo Molnar <mingo@...e.hu> wrote:
> > The pure per-CPU design would have to embedd the CPU ID the object is
> > attached to into the allocated object. If that is not feasible then
> > only the global hash remains i think.
>
> embedding the info shouldnt be /that/ hard in case of the SLAB: if the
> memleak info is at a negative offset from the allocated pointer. I.e.
> that if kmalloc() returns 'ptr', the memleak info could be at
> ptr-sizeof(memleak_info). That way you dont have to know the size of the
> object beforehand and there's absolutely no need for a global hash of
> any sort.

It would probably need to be just a pointer embedded in the allocated
block. With the current design, the memleak objects have a lifetime
longer than the tracked block. This is mainly to avoid long locking
during memory scanning and reporting.

> (it gets a bit more complex for page aligned allocations for the buddy
> and for vmalloc - but that could be solved by adding one extra pointer
> into struct page. [...]

This still leaves the issue of marking objects as not being leaks or
being of a different type. This is done by calling memleak_* functions
at the allocation point (outside allocator) where only the pointer is
known. In the vmalloc case, it would need to call find_vm_area. This
might not be a big problem, only that memory resources are no longer
treated in a unified way by kmemleak (and might not be trivial to add
support for new allocators).

> [...] That is a far more preferable cost than the
> locking/cache overhead of a global hash.)

A global hash would need to be re-built for every scan (and destroyed
afterwards), making this operation longer since the pointer values
together with their aliases (resulted from using container_of) are
added to the hash.

I understand the benefits but I personally favor simplicity over
performance, especially for code used as a debugging tool. DEBUG_SLAB
already introduces an overhead by poisoning the allocated blocks.
Generating the backtrace and filling in the memleak objects for every
allocation is another overhead. Global structures are indeed a
scalability problem but for a reasonable number of CPUs their overhead
might not be that big. Anyway, I can't be sure without some
benchmarking and this is probably highly dependent on the system
(caches, snoop control unit etc.).

-- 
Catalin
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