On Fri, Feb 07, 2025 at 09:35:27AM -0500, Zi Yan wrote: > On 7 Feb 2025, at 9:25, Matthew Wilcox wrote: > > 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). __filemap_add_folio() doesn't need to fracture like it currently does; it can do the same minimum split. The situation is that we've got a shadow entry which covers 2^n slots, and now we want to add a folio which only covers 2^m slots with m < n. Leaving n-m shadow entries in the tree with orders ranging from m to n-1 makes more sense than the eager split. shmem is the same, except that it's storing swap entries instead of shadow entries.
