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

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Message-Id: <20220629193006.77e9f071a5940e882c459cdd@linux-foundation.org>
Date:   Wed, 29 Jun 2022 19:30:06 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, dave.hansen@...el.com,
        Joerg Roedel <jroedel@...e.de>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc

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