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

[Vlastimil Babka]

On 10/18/21 05:38, 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).

This size limitation seems like an unnecessary gotcha. Couldn't we make
these large allocations in slob_alloc_node() (that use slob_new_pages()
directly) similar enough to large kmalloc() ones, so that kfree() can
recognize them and free properly? AFAICS it might mean just adding
__GFP_COMP to make sure there's a compound order stored, as these already
don't seem to set PageSlab.

> The main reason for this change is to simplify SLOB a little bit, make
> it a bit easier to debug whenever something goes wrong.

I would say the main reason is to simplify the slab API and guarantee that
both kmem_cache_alloc() and kmalloc() can be freed by kfree().

We should also update the comments at top of slob.c to reflect the change.
And Documentation/core-api/memory-allocation.rst (the last paragraph).

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

2.5% increase, hopefully acceptable.

Thanks!