Re: [PATCH] xarray: Add a BUG_ON() to ensure caller is not sibling

[Zi Yan]

On 3 Jun 2025, at 8:59, Dev Jain wrote:

> On 03/06/25 5:47 pm, Zi Yan wrote:
>> On 3 Jun 2025, at 3:58, David Hildenbrand wrote:
>>
>>> On 03.06.25 07:23, Dev Jain wrote:
>>>> On 02/06/25 8:33 pm, Zi Yan wrote:
>>>>> On 29 May 2025, at 23:44, Dev Jain wrote:
>>>>>
>>>>>> On 30/05/25 4:17 am, Zi Yan wrote:
>>>>>>> On 28 May 2025, at 23:17, Dev Jain wrote:
>>>>>>>
>>>>>>>> On 28/05/25 10:42 pm, Zi Yan wrote:
>>>>>>>>> On 28 May 2025, at 7:31, Dev Jain wrote:
>>>>>>>>>
>>>>>>>>>> Suppose xas is pointing somewhere near the end of the multi-entry batch.
>>>>>>>>>> Then it may happen that the computed slot already falls beyond the batch,
>>>>>>>>>> thus breaking the loop due to !xa_is_sibling(), and computing the wrong
>>>>>>>>>> order. Thus ensure that the caller is aware of this by triggering a BUG
>>>>>>>>>> when the entry is a sibling entry.
>>>>>>>>> Is it possible to add a test case in lib/test_xarray.c for this?
>>>>>>>>> You can compile the tests with “make -C tools/testing/radix-tree”
>>>>>>>>> and run “./tools/testing/radix-tree/xarray”.
>>>>>>>> Sorry forgot to Cc you.
>>>>>>>> I can surely do that later, but does this patch look fine?
>>>>>>> I am not sure the exact situation you are describing, so I asked you
>>>>>>> to write a test case to demonstrate the issue. :)
>>>>>> Suppose we have a shift-6 node having an order-9 entry => 8 - 1 = 7 siblings,
>>>>>> so assume the slots are at offset 0 till 7 in this node. If xas->xa_offset is 6,
>>>>>> then the code will compute order as 1 + xas->xa_node->shift = 7. So I mean to
>>>>>> say that the order computation must start from the beginning of the multi-slot
>>>>>> entries, that is, the non-sibling entry.
>>>>> Got it. Thanks for the explanation. It will be great to add this explanation
>>>>> to the commit log.
>>>>>
>>>>> I also notice that in the comment of xas_get_order() it says
>>>>> “Called after xas_load()” and xas_load() returns NULL or an internal
>>>>> entry for a sibling. So caller is responsible to make sure xas is not pointing
>>>>> to a sibling entry. It is good to have a check here.
>>>>>
>>>>> In terms of the patch, we are moving away from BUG()/BUG_ON(), so I wonder
>>>>> if there is a less disruptive way of handling this. Something like return
>>>>> -EINVAL instead with modified function comments and adding a comment
>>>>> at the return -EIVAL saying something like caller needs to pass
>>>>> a non-sibling entry.
>>>> What's the reason for moving away from BUG_ON()?
>>> BUG_ON is in general a bad thing. See Documentation/process/coding-style.rst and the history on the related changes for details.
>>>
>>> Here, it is less critical than it looks.
>>>
>>> XA_NODE_BUG_ON is only active with XA_DEBUG.
>>>
>>> And XA_DEBUG is only defined in
>>>
>>> tools/testing/shared/xarray-shared.h:#define XA_DEBUG
>>>
>>> So IIUC, it's only active in selftests, and completely inactive in any kernel builds.
>> Oh, I missed that. But that also means this patch becomes a nop in kernel
>
> Yes, but given other places are there with XA_NODE_BUG_ON(), I believe
> this patch has some value :)

Sure. Can you please also add something like below to the function comment?
“The xas cannot be a sibling entry, otherwise the result will be wrong”
It saves other’s time to infer it from the added XA_NODE_BUG_ON().

Thanks.

Best Regards,
Yan, Zi