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

[Christoph Lameter]

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.