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

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Date: Fri, 28 Jun 2024 18:53:24 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Kees Cook <kees@...nel.org>, Boqun Feng <boqun.feng@...il.com>
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 6/28/24 5:47 PM, Kees Cook wrote:
> On Thu, Jun 27, 2024 at 10:35:36PM -0700, Boqun Feng wrote:
>> On Thu, Jun 20, 2024 at 10:43:39PM +0200, Vlastimil Babka wrote:
>> > 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.
> 
> I can drop the "align" argument. Do we want to hard-code a
> per-cache-size alignment for the caches in a kmem_buckets collection?

I think you can drop it as a single value is really ill suited for a
collection of different sizes.

As for Rust's requirements we could consider whether to add a special flag
if they use own bucket, or just implement the rules for non-power-of-two
size globally. It should be feasible as I think in the non-debug caches it
shouldn't in fact change the existing layout, which is the same situation as
when we codified the power-of-two caches alignment as guaranteed.