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

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Date: Fri, 28 Jun 2024 11:34:35 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Boqun Feng <boqun.feng@...il.com>, 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 Fri, Jun 28, 2024 at 11:17 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 6/28/24 11:06 AM, Alice Ryhl wrote:
> >> >>
> >> >
> >> > 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?
> >
> > Rust's allocator API can take any size and alignment as long as:
> >
> > 1. The alignment is a power of two.
> > 2. The size is non-zero.
> > 3. When you round up the size to the next multiple of the alignment,
> > then it must not overflow the signed type isize / ssize_t.
> >
> > What happens right now is that when Rust wants an allocation with a
> > higher alignment than ARCH_SLAB_MINALIGN, then it will increase size
> > until it becomes a power of two so that the power-of-two guarantee
> > gives a properly aligned allocation.
>
> So am I correct thinking that, if the cache of size 96 bytes guaranteed a
> 32byte alignment, and 192 bytes guaranteed 64byte alignment, and the rest of
> sizes with the already guaranteed power-of-two alignment, then on rust side
> you would only have to round up sizes to the next multiples of the alignemnt
> (rule 3 above) and that would be sufficient?
>  Abstracting from the specific sizes of 96 and 192, the guarantee on kmalloc
> side would have to be - guarantee alignment to the largest power-of-two
> divisor of the size. Does that sound right?
>
> Then I think we could have some flag for kmem_buckets creation that would do
> the right thing.

If kmalloc/krealloc guarantee that an allocation is aligned according
to the largest power-of-two divisor of the size, then the Rust
allocator would definitely be simplified as we would not longer need
this part:

if layout.align() > bindings::ARCH_SLAB_MINALIGN {
    // The alignment requirement exceeds the slab guarantee, thus try
to enlarge the size
    // to use the "power-of-two" size/alignment guarantee (see
comments in `kmalloc()` for
    // more information).
    //
    // Note that `layout.size()` (after padding) is guaranteed to be a
multiple of
    // `layout.align()`, so `next_power_of_two` gives enough alignment
guarantee.
    size = size.next_power_of_two();
}

We would only need to keep the part that rounds up the size to a
multiple of the alignment.

Alice