[RFC PATCH v3 0/2] Fix storing in XArray check_split tests

Ackerley Tng
2026-02-23T07:34:27Z

Hi,

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?

Previous versions:

+ RFC v3: Cleaned up commits and subject references (sorry for the noise!)
+ RFC v2: Rebased on Linux 7.0-rc1 (https://lore.kernel.org/all/cover.1771831180.git.ackerleytng@google.com/T/)
+ RFC v1: https://lore.kernel.org/all/720e32d8e185d5c82659bbdede05e87b3318c413.1769818406.git.ackerleytng@google.com/

[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(-)

base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
--
2.53.0.345.g96ddfc5eaa-goog