Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc

[Andrew Morton]

On Thu, 30 Jun 2022 09:47:15 +0800 Feng Tang <feng.tang@xxxxxxxxx> wrote:

> kmalloc's API family is critical for mm, with one shortcoming that
> its object size is fixed to be power of 2. When user requests memory
> for '2^n + 1' bytes, actually 2^(n+1) bytes will be allocated, so
> in worst case, there is around 50% memory space waste.
> 
> We've met a kernel boot OOM panic, and from the dumped slab info:
> 
>     [   26.062145] kmalloc-2k            814056KB     814056KB
> 
> >From debug we found there are huge number of 'struct iova_magazine',
> whose size is 1032 bytes (1024 + 8), so each allocation will waste
> 1016 bytes. Though the issue is solved by giving the right(bigger)
> size of RAM, it is still better to optimize the size (either use
> a kmalloc friendly size or create a dedicated slab for it).

Well that's nice, and additional visibility is presumably a good thing.

But what the heck is going on with iova_magazine?  Is anyone looking at
moderating its impact?