On Sat, Dec 11, 2021 at 11:55:27AM +0000, Hyeonggon Yoo wrote: > On Fri, Dec 10, 2021 at 07:26:11PM +0100, Vlastimil Babka wrote: > > On 12/10/21 17:37, Hyeonggon Yoo wrote: > > > On Wed, Dec 01, 2021 at 07:15:08PM +0100, Vlastimil Babka wrote: > > >> With a struct slab definition separate from struct page, we can go further and > > >> define only fields that the chosen sl*b implementation uses. This means > > >> everything between __page_flags and __page_refcount placeholders now depends on > > >> the chosen CONFIG_SL*B. > > > > > > When I read this patch series first, I thought struct slab is allocated > > > separately from struct page. > > > > > > But after reading it again, It uses same allocated space of struct page. > > > > Yes. Allocating it elsewhere is something that can be discussed later. It's > > not a simple clear win - more memory used, more overhead, complicated code... > > > > Right. That is a something that can be discussed, > But I don't think there will be much win. Oh no, there's a substantial win. If we can reduce struct page to a single pointer, that shrinks it from 64 bytes/4k to 8 bytes/4k. Set against that, you have to allocate the struct folio / struct slab / ... but then it's one _per allocation_ rather than one per page. So for an order-2 allocation, it takes 32 bytes + 64 bytes (= 96 bytes) rather than 4*64 = 256 bytes. It's an even bigger win for larger allocations, and it lets us grow the memory descriptors independently of each other. But it's also a substantial amount of work, so don't expect us to get there any time soon. Everything currently using struct page needs to be converted to use another type, and that's just the pre-requisite step. Some more thoughts on this here: https://lore.kernel.org/linux-mm/YXcLqcFhDq3uUwIj@xxxxxxxxxxxxxxxxxxxx/ > > Yeah. Also whatever aliases with compound_head must not have bit zero set as > > that means a tail page. > > > > Oh I was missing that. Thank you. > > Hmm then in struct slab, page->compound_head and slab->list_head (or > slab->rcu_head) has same offset. And list_head / rcu_head both store pointers. > > then it has a alignment requirement. (address saved in list_head/rcu_head > should be multiple of 2) > > Anyway, it was required long time before this patch, > so it is not a problem for this patch. Yes, that's why there's an assert that the list_heads all line up. This requirement will go away if we do get separately allocated memory descriptors (because that bottom bit is no longer PageTail).
