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

[Zi Yan]

On 16 Mar 2026, at 12:23, David Hildenbrand (Arm) wrote:

> On 2/23/26 08:34, Ackerley Tng wrote:
>> 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?
>
> Zi, do you have any familiarity with that code and could help?

Not much. But I used lib/test_xarray.c to did a test:

1. initialize an xarray with order 6 and set entry to 0,

2. add a new xas like XA_STATE(xas0, xa, 0);
3. xas_store(&xas0, xa_mk_value(32));

4. add a new xas like XA_STATE(xas0, xa, 16);
5. xas_store(&xas0, xa_mk_value(48));

The outcome is that xas_store() does not change xarray structure,
namely the orders are preserved. No issue is present.

After 2 and 3, the xarray is still order 6, but its 0-63 entries (all order-6)
are changed from 0 to 32.
After 4 and 5, the xarray is still order 6, but its 0-63 entries
are changed from 32 to 48.

I will need to dig into the code more to explain how xas_store() works.

Best Regards,
Yan, Zi