mm: mempool: fix crash in mempool_free() for zero-minimum pools

mm: mempool: fix crash in mempool_free() for zero-minimum pools

The mempool wake-up fix introduced in commit a5867a218d7c ("mm: mempool:
fix wake-up edge case bug for zero-minimum pools") inlined the
add_element() logic in mempool_free() to return the element to the
zero-minimum pool:

pool->elements[pool->curr_nr++] = element;

This causes crash, because mempool_init_node() does not initialize with
real allocation for zero-minimum pool, it only returns ZERO_SIZE_PTR to
the elements array which is unable to be dereferenced, and the
pre-allocation of this array never happened since the while test:

while (pool->curr_nr < pool->min_nr)

can never be satisfied as min_nr is zero, so the pool does not actually
reserve any buffer, the only way so far is to call alloc_fn() to get
buffer from SLUB, but if the memory is under high pressure the alloc_fn()
could never get any buffer, the waiting thread would be in an indefinite
loop of wake-sleep in a period until there is free memory to get.

This patch changes mempool_init_node() to allocate 1 element for the
elements array of zero-minimum pool, so that the pool will have reserved
buffer to use.  This will fix the crash issue and let the waiting thread
can get the reserved element when alloc_fn() failed to get buffer under
high memory pressure.

Also modify add_element() to support zero-minimum pool with simplifying
codes of zero-minimum handling in mempool_free().

Link: https://lkml.kernel.org/r/e01f00f3-58d9-4ca7-af54-bfa42fec9527@suse.com
Fixes: a5867a218d7c ("mm: mempool: fix wake-up edge case bug for zero-minimum pools")
Signed-off-by: Yadan Fan <ydfan@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>