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

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Date: Fri, 28 Jun 2024 10:40:09 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Kees Cook <kees@...nel.org>, "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 6/28/24 7:35 AM, Boqun Feng wrote:
> 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.

Hm yeah we only have guarantees for power-or-2 allocations.

>> 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.

How flexible do you need those alignments to be? Besides the power-of-two
guarantees, we currently have only two odd sizes with 96 and 192. If those
were guaranteed to be aligned 32 bytes, would that be sufficient? Also do
you ever allocate anything smaller than 32 bytes then?

To summarize, if Rust's requirements can be summarized by some rules and
it's not completely ad-hoc per-allocation alignment requirement (or if it
is, does it have an upper bound?) we could perhaps figure out the creation
of rust-specific kmem_buckets to give it what's needed?

> 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.
>>