linux-kernel - Re: [PATCH v2] xarray: Add a BUG_ON() to ensure caller is not sibling

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Message-ID: <3D370CEA-9F26-4E6E-B93E-D26F2A4A222B@nvidia.com>
Date: Wed, 04 Jun 2025 09:40:49 -0400
From: Zi Yan <ziy@...dia.com>
To: Dev Jain <dev.jain@....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, aneesh.kumar@...nel.org
Subject: Re: [PATCH v2] xarray: Add a BUG_ON() to ensure caller is not sibling

On 4 Jun 2025, at 0:15, 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. For example, 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. Therefore, the order computation must start
> from the beginning of the multi-slot entries, that is, the non-sibling
> entry. Thus ensure that the caller is aware of this by triggering a BUG
> when the entry is a sibling entry. Note that this BUG_ON() is only
> active while running selftests, so there is no overhead in a running
> kernel.
>
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---
> v1->v2:
>  - Expand changelog, add comment
>
> Based on Torvalds' master branch.
>
>  lib/xarray.c | 3 +++
>  1 file changed, 3 insertions(+)
>

The added comment is also clarifying the function requirement. Thanks.

Acked-by: Zi Yan <ziy@...dia.com>

--
Best Regards,
Yan, Zi