[RFC PATCH 0/4] Extend xas_split* to support splitting arbitrarily large entries

[Ackerley Tng <ackerleytng@google.com>]

guest_memfd is planning to store huge pages in the filemap, and
guest_memfd's use of huge pages involves splitting of huge pages into
individual pages. Splitting of huge pages also involves splitting of
the filemap entries for the pages being split.

To summarize the context of how these patches will be used,

+ guest_memfd stores huge pages (up to 1G pages) in the filemap.
+ During folio splitting, guest_memfd needs split the folios, and
  approaches that by first splitting the filemap (XArray) entries that
  the folio occupies, and then splitting the struct folios themselves.
+ Splitting from a 1G to 4K folio requires splitting an entry in a
  shift-18 XArray node to a shift-0 node in the xarray, which goes
  beyond 2 levels of XArray nodes, and is currently not supported.

This work-in-progress series at [1] shows the context of how these
patches for XArray entry splitting will be used.

[1] https://github.com/googleprodkernel/linux-cc/tree/wip-gmem-conversions-hugetlb-restructuring

This patch series extends xas_split_alloc() to allocate enough nodes
for splitting an XArray node beyond 2 levels, and extends xas_split()
to use the allocated nodes in a split beyond 2 levels.

Merging of XArray entries can be performed with xa_store_order() at
the original order, and hence no change to the XArray library is
required.

xas_destroy() cleans up any allocated and unused nodes in struct
xa_state, and so no further changes are necessary there.

Please let me know

+ If this extension is welcome
+ Your thoughts on the approach: is it too many nodes to allocate at
  once? Would a recursive implementation be preferred?
+ If there are any bugs, particularly around how xas_split() interacts
  with LRU


Thank you!


Ackerley Tng (4):
  XArray: Initialize nodes while splitting instead of while allocating
  XArray: Update xas_split_alloc() to allocate enough nodes to split
    large entries
  XArray: Support splitting for arbitrarily large entries
  XArray: test: Increase split order test range in check_split()

 lib/test_xarray.c |   6 +-
 lib/xarray.c      | 210 ++++++++++++++++++++++++++++++++++------------
 2 files changed, 162 insertions(+), 54 deletions(-)

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2.52.0.rc1.455.g30608eb744-goog