[PATCH] realloc: Return unchanged if request is within usable size

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Siddhesh,

I think the idea of having a fast path in realloc is good since it is a more standard
interface than malloc_usable_size, so improving performance for obvious cases
where we can return the original pointer makes sense. It will help badly written
code that does a realloc when growing an array 1 element at a time.

However the proposed patch will block realloc from ever releasing memory...
We do need to handle the case where the size is smaller than the usable size since
we'd still want to free the unused portion if it is a significant fraction of the total size.
Note the mmap code always tries to remap before checking the new size is smaller
than usable size - so will end up remapping even if you downsize by one page.

Cheers,
Wilco