Re: [PATCH 0/2] Minimize xa_node allocation during xarry split

[Zi Yan]

On 12 Feb 2025, at 22:43, 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.
>
> Let me know your comments.

The patch is on top of mm-everything-2025-02-13-02-18.

>
>
> [1] https://lore.kernel.org/linux-mm/Z6YX3RznGLUD07Ao@xxxxxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/20250211155034.268962-2-ziy@xxxxxxxxxx/
>
> Zi Yan (2):
>   mm/filemap: use xas_try_split() in __filemap_add_folio().
>   mm/shmem: use xas_try_split() in shmem_split_large_entry().
>
>  include/linux/xarray.h |  7 +++++++
>  lib/xarray.c           | 25 +++++++++++++++++++++++
>  mm/filemap.c           | 46 +++++++++++++++++-------------------------
>  mm/shmem.c             | 43 +++++++++++++++------------------------
>  4 files changed, 67 insertions(+), 54 deletions(-)
>
> -- 
> 2.47.2


Best Regards,
Yan, Zi