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

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Message-ID: <a3311974-30ae-42b6-9f26-45e769a67522@arm.com>
Date: Tue, 3 Jun 2025 10:53:31 +0530
From: Dev Jain <dev.jain@....com>
To: 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, david@...hat.com, anshuman.khandual@....com,
 ryan.roberts@....com
Subject: Re: [PATCH] xarray: Add a BUG_ON() to ensure caller is not sibling


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()? I would think that it is
better that we don't have any overhead without the relevant debug config.
Also, returning any negative return value seems more disruptive :) we will
have to change all the callers to handle that, and in turn, handle that
for their callers, and so on.

>
> Best Regards,
> Yan, Zi