[RFC PATCH v1 0/2] Fix storing in XArray check_split tests

[Ackerley Tng <ackerleytng@google.com>]

Hi,

(resending to fix Message-ID, also gently nudging for feedback!)

I hit an assertion while making some modifications to
lib/test_xarray.c [1] and I believe this is the fix.

In check_split, the tests split the XArray node and then store values
after the split to verify that splitting worked. While storing and
retrieval works as expected, the node's metadata, specifically
node->nr_values, is not updated correctly.

This led to the assertion being hit in [1], since the storing process
did not increment node->nr_values sufficiently, while the erasing
process assumed the fully-incremented node->nr_values state.

Would like to check my understanding on these:

1. In the multi-index xarray world, is node->nr_values definitely the
   total number of values *and siblings* in the node?

2. IIUC xas_store() has significantly different behavior when entry is
   NULL vs non-NULL: when entry is NULL, xas_store() does not make
   assumptions on the number of siblings and erases all the way till
   the next non-sibling entry. This sounds fair to me, but it's also
   kind of surprising that it is differently handled when entry is
   non-NULL, where xas_store() respects xas->xa_sibs.

3. If xas_store() is dependent on its caller to set up xas correctly
   (also sounds fair), then there are places where xas_store() is
   used, like replace_page_cache_folio() or
   migrate_huge_page_move_mapping(), where xas is set up assuming 0
   order pages. Are those buggy?

[1] https://lore.kernel.org/all/20251028223414.299268-1-ackerleytng@google.com/

Ackerley Tng (2):
  XArray tests: Fix check_split tests to store correctly
  XArray tests: Verify xa_erase behavior in check_split

 lib/test_xarray.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

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2.53.0.rc1.225.gd81095ad13-goog