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

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

On 2022-11-28 04:22, Wilco Dijkstra wrote:
> 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...

Thanks for pointing that out, I missed it.

> 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.

I'll make this a function of trim_threshold so that the reuse happens 
only if the difference between the request and usable size is less than 
the trim_threshold.

> 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.

I don't see that; I've put the musable check before the 
chunk_is_mmapped() check.  What am I missing?

Thanks,
Sid