realloc: Limit chunk reuse to only growing requests [BZ #30579]
The trim_threshold is too aggressive a heuristic to decide if chunk reuse is OK for reallocated memory; for repeated small, shrinking allocations it leads to internal fragmentation and for repeated larger allocations that fragmentation may blow up even worse due to the dynamic nature of the threshold. Limit reuse only when it is within the alignment padding, which is 2 * size_t for heap allocations and a page size for mmapped allocations. There's the added wrinkle of THP, but this fix ignores it for now, pessimizing that case in favor of keeping fragmentation low. This resolves BZ #30579. Signed-off-by:Siddhesh Poyarekar <siddhesh@sourceware.org> Reported-by:
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