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
