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

[Vlastimil Babka <vbabka@suse.cz>]

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.

> Alice