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

[David Hildenbrand (Arm)]

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?

Thanks!

-- 
Cheers,

David