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

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Message-ID: <D5EDD20A-03A2-4CEA-884F-D1E48875222B@nvidia.com>
Date: Tue, 03 Jun 2025 08:17:57 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Dev Jain <dev.jain@....com>, 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 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
builds.

Best Regards,
Yan, Zi