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

Rustam Kovhaev
2021-10-19T01:22:34Z

On Mon, Oct 18, 2021 at 11:22:46AM +0200, Vlastimil Babka wrote:
> 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.

Thanks for the pointers, I'll send a new version.

> > 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).

OK, thank you!

> > 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!