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

[Andrew Morton]

On Wed,  4 Jun 2025 09:45:33 +0530 Dev Jain <dev.jain@xxxxxxx> 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.

Why check this thing in particular?  There are a zillion things we
could check...

> Note that this BUG_ON() is only
> active while running selftests, so there is no overhead in a running
> kernel.

hm, how do we know this?  Now and in the future?  xa_get_order() and
xas_get_order() have callers all over the place.