linux-kernel - Re: [PATCH v4] slob: add size header to all allocations

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Date:   Mon, 22 Nov 2021 10:22:28 +0100 (CET)
From:   Christoph Lameter <cl@...two.org>
To:     Rustam Kovhaev <rkovhaev@...il.com>
cc:     vbabka@...e.cz, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org, corbet@....net,
        djwong@...nel.org, david@...morbit.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, gregkh@...uxfoundation.org,
        viro@...iv.linux.org.uk, dvyukov@...gle.com
Subject: Re: [PATCH v4] slob: add size header to all allocations

On Sun, 21 Nov 2021, Rustam Kovhaev wrote:

> Let's prepend both kmalloc() and kmem_cache_alloc() allocations with the
> size header.
> It simplifies the slab API and guarantees that both kmem_cache_alloc()
> and kmalloc() memory could be freed by kfree().
>
> meminfo right after the system boot, x86-64 on xfs, without the patch:
> Slab:              34700 kB
>
> the same, with the patch:
> Slab:              35752 kB

> +#define SLOB_HDR_SIZE max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN)

Ok that is up to 128 bytes on some architectues. Mostly 32 or 64 bytes.

> @@ -307,6 +303,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node,
>  	unsigned long flags;
>  	bool _unused;
>
> +	size += SLOB_HDR_SIZE;

And every object now has this overhead? 128 bytes extra in extreme cases
per object?


> -	if (size < PAGE_SIZE - minalign) {
> -		int align = minalign;
> +	if (size < PAGE_SIZE - SLOB_HDR_SIZE) {
> +		int align = SLOB_HDR_SIZE;

And the object is also aligned to 128 bytes boundaries on some
architectures.

So a 4 byte object occupies 256 bytes in SLOB?

SLOB will no longer be a low memory overhead allocator then.