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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 586771FCF3 X-Spam-Score: -3.00 X-Spam-Level: X-Spam-Flag: NO X-Spamd-Result: default: False [-3.00 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; XM_UA_NO_VERSION(0.01)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; ARC_NA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[24]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; MIME_TRACE(0.00)[0:+]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; FREEMAIL_CC(0.00)[gmail.com,kernel.org,huaweicloud.com,linux.com,google.com,lge.com,dustri.org,linux-foundation.org,linux.dev,huawei.com,suug.ch,gondor.apana.org.au,vger.kernel.org,kvack.org]; RCVD_TLS_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:106:10:150:64:167:received]; TAGGED_RCPT(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; RCVD_VIA_SMTP_AUTH(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,imap1.dmz-prg2.suse.org:rdns] X-Rspamd-Action: no action X-Rspamd-Server: rspamd1.dmz-prg2.suse.org 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