On 26 Feb 2025, at 16:08, Zi Yan wrote: > Hi all, > > When splitting a multi-index entry in XArray from order-n to order-m, > existing xas_split_alloc()+xas_split() approach requires > 2^(n % XA_CHUNK_SHIFT) xa_node allocations. But its callers, > __filemap_add_folio() and shmem_split_large_entry(), use at most 1 xa_node. > To minimize xa_node allocation and remove the limitation of no split from > order-12 (or above) to order-0 (or anything between 0 and 5)[1], > xas_try_split() was added[2], which allocates > (n / XA_CHUNK_SHIFT - m / XA_CHUNK_SHIFT) xa_node. It is used > for non-uniform folio split, but can be used by __filemap_add_folio() > and shmem_split_large_entry(). > > xas_split_alloc() and xas_split() split an order-9 to order-0: > > --------------------------------- > | | | | | | | | | > | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | > | | | | | | | | | > --------------------------------- > | | | | > ------- --- --- ------- > | | ... | | > V V V V > ----------- ----------- ----------- ----------- > | xa_node | | xa_node | ... | xa_node | | xa_node | > ----------- ----------- ----------- ----------- > > xas_try_split() splits an order-9 to order-0: > --------------------------------- > | | | | | | | | | > | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | > | | | | | | | | | > --------------------------------- > | > | > V > ----------- > | xa_node | > ----------- > > xas_try_split() is designed to be called iteratively with n = m + 1. > xas_try_split_mini_order() is added to minmize the number of calls to > xas_try_split() by telling the caller the next minimal order to split to > instead of n - 1. Splitting order-n to order-m when m= l * XA_CHUNK_SHIFT > does not require xa_node allocation and requires 1 xa_node > when n=l * XA_CHUNK_SHIFT and m = n - 1, so it is OK to use > xas_try_split() with n > m + 1 when no new xa_node is needed. > > xfstests quick group test passed on xfs and tmpfs. > > It is on top of Buddy allocator like (or non-uniform) > folio split V9[2], which is on top of mm-everything-2025-02-26-03-56. > > Changelog > === > From V2[3]: > 1. Fixed shmem_split_large_entry() by setting swap offset correct. > (Thank Baolin for the detailed review) > 2. Used updated xas_try_split() to avoid a bug when xa_node is allocated > by xas_nomem() instead of xas_try_split() itself. > > Let me know your comments. > > > [1] https://lore.kernel.org/linux-mm/Z6YX3RznGLUD07Ao@xxxxxxxxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/linux-mm/20250226210032.2044041-1-ziy@xxxxxxxxxx/ > [3] https://lore.kernel.org/linux-mm/20250218235444.1543173-1-ziy@xxxxxxxxxx/ Hi Andrew, Do you want me to resend this? This still applies cleanly on mm-everything-2025-03-07-07-55 plus V10. Thanks. Best Regards, Yan, Zi
