On Fri, Feb 07, 2025 at 09:11:39AM -0500, Zi Yan wrote: > Existing uniform split requires 2^(order % XA_CHUNK_SHIFT) xa_node allocations > during split, when the folio needs to be split to order-0. But non-uniform split > only requires at most 1 xa_node allocation. For example, to split an order-9 > folio, 8 xa_nodes are needed for uniform split, since the folio takes 8 > multi-index slots in the xarray. But for non-uniform split, only the slot > containing the given struct page needs a xa_node after the split. There will be > a 7 xa_node saving. > > Hi Matthew, > > Do you mind checking my statement above on xarray memory saving? And correct me > if I miss anything. Thanks. We currently have a bug where we can't split order-12 (or above) to order-0 (or anything in the range 0-5) as we'd need to allocate two layers of nodes, and the preallocation can't do that. As part of your series, I'd like to remove that limitation, so we'd need to allocate log_64(n - m) [ok, more complex than that, but ykwim]. So it's not quite "only allocate one node", but it's allocate O(log(current number of nodes needed to be allocated)). Makes sense?
