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

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Jun 28, 2024 at 11:17 AM Vlastimil Babka <vbabka@suse.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