On 7 Feb 2025, at 9:25, Matthew Wilcox wrote: > 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? Yes. To remove that order-12 limitation, do shmem_split_large_entry() and __filemap_add_folio() need some change as well? Both call xas_split_alloc(). But I do not know if they will see splitting order-12 to order-(0 to 5). Best Regards, Yan, Zi
