Re: [PATCH v5 4/6] mm/slab: Introduce kmem_buckets_create() and family

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Jun 20, 2024 at 10:43:39PM +0200, Vlastimil Babka wrote:
> On 6/20/24 8:54 PM, Kees Cook wrote:
> > On Thu, Jun 20, 2024 at 03:56:27PM +0200, Vlastimil Babka wrote:
> >> > @@ -549,6 +549,11 @@ void *kmem_cache_alloc_lru_noprof(struct kmem_cache *s, struct list_lru *lru,
> >> >  
> >> >  void kmem_cache_free(struct kmem_cache *s, void *objp);
> >> >  
> >> > +kmem_buckets *kmem_buckets_create(const char *name, unsigned int align,
> >> > +				  slab_flags_t flags,
> >> > +				  unsigned int useroffset, unsigned int usersize,
> >> > +				  void (*ctor)(void *));
> >> 
> >> I'd drop the ctor, I can't imagine how it would be used with variable-sized
> >> allocations.
> > 
> > I've kept it because for "kmalloc wrapper" APIs, e.g. devm_kmalloc(),
> > there is some "housekeeping" that gets done explicitly right now that I
> > think would be better served by using a ctor in the future. These APIs
> > are variable-sized, but have a fixed size header, so they have a
> > "minimum size" that the ctor can still operate on, etc.
> > 
> >> Probably also "align" doesn't make much sense since we're just
> >> copying the kmalloc cache sizes and its implicit alignment of any
> >> power-of-two allocations.
> > 
> > Yeah, that's probably true. I kept it since I wanted to mirror
> > kmem_cache_create() to make this API more familiar looking.
> 
> Rust people were asking about kmalloc alignment (but I forgot the details)

It was me! The ask is whether we can specify the alignment for the
allocation API, for example, requesting a size=96 and align=32 memory,
or the allocation API could do a "best alignment", for example,
allocating a size=96 will give a align=32 memory. As far as I
understand, kmalloc() doesn't support this.

> so maybe this could be useful for them? CC rust-for-linux.
> 

I took a quick look as what kmem_buckets is, and seems to me that align
doesn't make sense here (and probably not useful in Rust as well)
because a kmem_buckets is a set of kmem_caches, each has its own object
size, making them share the same alignment is probably not what you
want. But I could be missing something.

Regards,
Boqun

> >> I don't think any current kmalloc user would
> >> suddenly need either of those as you convert it to buckets, and definitely
> >> not any user converted automatically by the code tagging.
> > 
> > Right, it's not needed for either the explicit users nor the future
> > automatic users. But since these arguments are available internally,
> > there seems to be future utility,  it's not fast path, and it made things
> > feel like the existing API, I'd kind of like to keep it.
> > 
> > But all that said, if you really don't want it, then sure I can drop
> > those arguments. Adding them back in the future shouldn't be too
> > much churn.
> 
> I guess we can keep it then.
>