On 6/28/24 11:06 AM, Alice Ryhl wrote:

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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.

Alice