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

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Message-ID: <053ae9ec-1113-4ed8-9625-adf382070bc5@redhat.com>
Date: Tue, 3 Jun 2025 09:58:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, Zi Yan <ziy@...dia.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, anshuman.khandual@....com, ryan.roberts@....com
Subject: Re: [PATCH] xarray: Add a BUG_ON() to ensure caller is not sibling

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.

-- 
Cheers,

David / dhildenb