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

[Gavin Shan <shangw@linux.vnet.ibm.com>]

>> 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.
>

yep, I need improve my written English definitely :-)

>--
>
>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.
>
>--

Thank you very much, Dave. Let me merge your changelog into next version.

>
>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.

I didn't test this and I just catch the point while reading the source
code. By the way, I would like to know the popular utilities used for
memory testing. If you can share some information regarding that, that
would be great.

	- memory related benchmark testing utility.
	- some documents on Linux memory testing.

Thanks,
Gavin

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