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
