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

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Message-ID: <20211020114638.GA378758@kvm.asia-northeast3-a.c.our-ratio-313919.internal>
Date:   Wed, 20 Oct 2021 11:46:38 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Rustam Kovhaev <rkovhaev@...il.com>
Cc:     vbabka@...e.cz, 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 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.

> 
> 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()?

Thanks,
Hyeonggon

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