Re: "alloc_tag was not set" when running mm/ksft_hmm.sh

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/12/26 09:46, Alistair Popple wrote:
> On 2026-05-12 at 16:47 +1000, "David Hildenbrand (Arm)" <david@kernel.org> wrote...
>> On 5/12/26 03:28, Alistair Popple wrote:
>>> On 2026-05-12 at 02:38 +1000, Zenghui Yu <zenghui.yu@linux.dev> wrote...
>>>
>>> Thanks. I have reproduced it now that my fingers are skinnier.
>>>
>>>
>>> Well you know what they say about assumptions :) Although in this case
>>> __folio_freeze_and_split_unmapped() isn't called on the backing page anyway
>>> (it's called to split the ZONE_DEVICE page, not the page simulating device
>>> memory). 
>>
>> Now my brain hurts :)
> 
> I have never liked this bit of the HMM selftests. It has always made my brain
> hurt.
> 
>>> The problem is we're not splitting the tag associated with the backing
>>> page for the simulated memory.
>>>
>>> I came up with the below fix last night, but I suspect it will quite reasonably
>>> get NACKED on the basis of the symbol export so was looking at other solutions.
>>
>> I think there are other problems ...
>>
>>>
>>> The simulated memory should just be used like a bare physical address range. So
>>> there really is no reason for the backing page simulating device memory to be
>>> allocated as a higher order folio. Using the struct page to store some metadata
>>> for the simulated device is convenient though to avoid creating a test-specific
>>> data structure for this. So I am looking at going back to allocating the
>>> simulated backing memory as always order-0 pages in the test which is what it
>>> was prior to the introduction of large device pages, but that was causing a
>>> crash I'm yet to debug.
>>>
>>
>> ... such as doing a folio_page(folio_alloc()), followed by a __free_pages().
>>
>> Why are we even allocating folios here and manually splitting them?
>>
>> Looking at dmirror_devmem_folio_split(), aren't we using folios here for
>> something that ... is not a folio?
>>
>> Likely we really shouldn't be using folios here ... :)
> 
> Exactly my point, just more succinct :)
> 
> I just need to make it work without doing that.

Ok, great. You could also allocate a larger buddy page and just split it
manually through split_page.

split_page will even do the pgalloc_tag_split :)

-- 
Cheers,

David