Re: [PATCH v3 08/13] selftests/mm: ensure destination is hugetlb-backed in hugepage-mremap

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

On 4/1/26 22:39, Sayali Patil wrote:
> 
> 
> On 01/04/26 20:10, Lorenzo Stoakes (Oracle) wrote:
>> On Wed, Apr 01, 2026 at 04:21:55PM +0200, David Hildenbrand (Arm) wrote:
>>
>> OK so digging in:
>>
>> mremap -> ... -> vrm_set_new_addr() -> get_unmapped_area() -> ... (in
>> ppc arch
>> code) -> slice_get_unmapped_area():
>>
>> unsigned long slice_get_unmapped_area(unsigned long addr, unsigned
>> long len,
>>                       unsigned long flags, unsigned int psize,
>>                       int topdown)
>> {
>>     ...
>>     /* bunch of checks */
>>
>>     /* If we have MAP_FIXED and failed the above steps, then error out */
>>     if (fixed)
>>         return -EBUSY;
>>
>>     ...
>> }
>>
>> Is presumably where we hit the issue.
>>
>>>
>>> That is weird. An mremap(MREMAP_FIXED) is really just an munmap() +
>>> move.
>>
>> Yeah the weird bit I guess is that we _still_ invoke
>> get_unmapped_area() but
>> with MAP_FIXED set to indicate that we want the specific address, so it's
>> subject to the above checks.
>>
>>>
>>> Are we sure this is not some actual problem in the hugetlb
>>> implementation?
>>
>> It seems the 'slices' check sees if the _target address_ has an
>> equivalent page
>> size, presumably hugetlb-mandated, and fails if they're not
>> equivalent, so this
>> change is just accounting for that.
>>
> Yes, this change accounts for that by ensuring the destination is
> created with MAP_HUGETLB so it has the same page size as the source.

Okay, weird, so it's the right thing to do to cover all odd arch behavior.

>>
>>>
>>>
>>> But then the test suddenly requires more hugetlb pages, no? I don't see
>>> a good reason for the MAP_POPULATE, really. It will be discarded
>>> either way.
>>
>> Yeah I'm not sure about the MAP_POPULATE being all that important here.
>>
> As far as I understand, without MAP_POPULATE, memory accesses would
> trigger userfaults, and since the test is single-threaded and has no
> background handler for the uffd, it would deadlock. MAP_POPULATE ensures
> the test runs correctly by prefaulting all pages, but please let me know
> if I’m mistaken.

So you are saying the test would deadlock if you are not adding
MAP_POPULATE? If so, please double check if that is actually the case.

And if it's actually the case, please carefully document that in the
patch description, and probably as a comment above the MAP_POPULATE usage.

-- 
Cheers,

David