linux-kernel - Re: [PATCH v5 4/6] mm/slab: Introduce kmem_buckets

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Date: Thu, 20 Jun 2024 22:43:39 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Kees Cook <kees@...nel.org>
Cc: "GONG, Ruiqi" <gongruiqi@...weicloud.com>,
 Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 jvoisin <julien.voisin@...tri.org>, Andrew Morton
 <akpm@...ux-foundation.org>, Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Xiu Jianfeng <xiujianfeng@...wei.com>,
 Suren Baghdasaryan <surenb@...gle.com>,
 Kent Overstreet <kent.overstreet@...ux.dev>, Jann Horn <jannh@...gle.com>,
 Matteo Rizzo <matteorizzo@...gle.com>, Thomas Graf <tgraf@...g.ch>,
 Herbert Xu <herbert@...dor.apana.org.au>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-hardening@...r.kernel.org, netdev@...r.kernel.org,
 rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v5 4/6] mm/slab: Introduce kmem_buckets_create() and
 family

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

>> 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.