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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/16/26 17:49, Zi Yan wrote:
> 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.

Zi,

we discussed this topic with Willy in the THP cabal call. I did not get
all the details, do you remember our conclusion?

(I can try getting access to the recording)

-- 
Cheers,

David