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
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