Re: [PATCH v2 2/3] mm/sparse: fix possible memory leak

[Dave Hansen <dave@linux.vnet.ibm.com>]

On 06/27/2012 09:36 AM, Gavin Shan wrote:
> With CONFIG_SPARSEMEM_EXTREME, the root memory section descriptors
> are allocated by slab or bootmem allocator. Also, the descriptors
> might have been allocated and initialized during the hotplug path.
> However, the memory chunk allocated in current implementation wouldn't
> be put into the available pool if that has been allocated. The situation
> will lead to memory leak.

I've read this changelog about ten times and I'm still not really clear
what the bug is here.

--

sparse_index_init() is designed to be safe if two copies of it race.  It
uses "index_init_lock" to ensure that, even in the case of a race, only
one CPU will manage to do:

	mem_section[root] = section;

However, in the case where two copies of sparse_index_init() _do_ race,
the one that loses the race will leak the "section" that
sparse_index_alloc() allocated for it.  This patch fixes that leak.

--

Technically, I'm not sure that we can race during the time when we'd be
using bootmem.  I think we do all those initializations single-threaded
at the moment, and we'd finish them before we turn the slab on.  So,
technically, we probably don't need the bootmem stuff in
sparse_index_free().  But, I guess it doesn't hurt, and it's fine for
completeness.

Gavin, have you actually tested this in some way?  It looks OK to me,
but I worry that you've just added a block of code that's exceedingly
unlikely to get run.

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