linux-kernel - Re: [PATCH] slob: add size header to all allocations

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Date:   Thu, 21 Oct 2021 19:36:26 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Rustam Kovhaev <rkovhaev@...il.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        djwong@...nel.org, david@...morbit.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk,
        dvyukov@...gle.com
Subject: Re: [PATCH] slob: add size header to all allocations

On 10/20/21 13:46, Hyeonggon Yoo wrote:
> On Sun, Oct 17, 2021 at 08:38:41PM -0700, Rustam Kovhaev wrote:
>> Let's prepend all  allocations of (PAGE_SIZE - align_offset) and less
>> with the size header. This way kmem_cache_alloc() memory can be freed
>> with kfree() and the other way around, as long as they are less than
>> (PAGE_SIZE - align_offset).
> 
> Hello Rustam, I measured its impact on memory usage on
> tiny kernel configuration as SLOB is used in very small machine.
> 
> on x86 32 bit + tinyconfig:
>     Before:
>     Slab:                668 kB
> 
>     After:
>     Slab:                688~692 kB
> 
> it adds 20~24kB.

Thanks for the measurement. That's 3.5% increase.

> 
>> 
>> The main reason for this change is to simplify SLOB a little bit, make
>> it a bit easier to debug whenever something goes wrong.
>>
> 
> It seems acceptable But I wonder it is worth to increase memory usage
> to allow freeing kmem_cache_alloc-ed objects by kfree()?

Not for the reason above, but for providing a useful API guarantee
regardless of selected slab allocator IMHO yes.

> Thanks,
> Hyeonggon
> 
>> meminfo right after the system boot, without the patch:
>> Slab:              35500 kB
>> 
>> the same, with the patch:
>> Slab:              36396 kB
>> 
>