Re: [PATCH RESEND 0/9] selftests/mm: improve mremap_test

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

On 9/29/26 09:32, Kalesh Singh wrote:
> On Mon, Sep 28, 2026 at 10:54 PM Sarthak Sharma <sarthak.sharma@arm.com> wrote:
>>
>>
>>
>> On 9/24/26 10:30 AM, Sarthak Sharma wrote:
>>> This series fixes several correctness issues in mremap_test and
>>> simplifies and strengthens its data validation.
>>>
>>> Patch 1 converts the mremap_test to use kselftest helpers, removes
>>> manual tracking of failed tests and corrects some spelling errors.
>>>
>>> Patches 2 to 5 fix userfaultfd skipping, unexpected mremap success
>>> handling, multi-VMA data validation and failure reporting when data
>>> corruption is detected.
>>>
>>> Patch 6 removes the randomization and uses a simple pattern based approach.
>>>
>>> Patch 7 removes perf tests and timing infrastructure.
>>>
>>> Patch 8 removes validation threshold and always validates complete mappings.
>>>
>>> Patch 9 strengthens the multi VMA validation by also checking the
>>> mapping state of holes after remapping.
>>
>> Hello everyone! Just wanted to check if someone has had a chance to look
>> at the series.
>>
>> Also, Sashiko has a concern [1], and I had the same while posting the
>> series. I hope I can get some opinion from the community.
>>
>> Currently, for PUD remap tests, we allocate a source mapping of 2GB. We
>> only fault in the first threshold_mb amount of memory, remap the whole
>> region, and validate the already faulted threshold_mb amount of memory,
>> which is, by default, equal to 4MB and can be changed by the user by
>> supplying a command line option.
>>
>> Since we plan to remove all command line options from teh selftests, I
>> removed threshold_mb altogether, following some discussion on the list
>> [2]. This would now cause a 2GB source mapping, and its contents copied
>> from another 2GB buffer. This causes the process to have a 4GB RSS.

Yeah, that's a lot for small CI systems indeed.

>> Sashiko says that this can cause OOM killing in small CI machines.
>>
>> Would it be okay to keep a 4GB RSS in this case, or should we find some
>> other way of validating a part of the whole range instead?
> 
> Hi Sarthak,
> 
> IIRC when I initially introduced the test, John was concerned that
> validating the whole range would significantly increase the duration
> of the mm selftests; this is why the threshold was introduced. Please
> check how much it increases if we validate the full range (with
> David's suggestions) and if it's no longer a concern from other folks.
> I am fine with removing the threshold.

As discussed off-list, I guess it makes more sense to validate a couple of pages
at the beginning, the middle and the end?

-- 
Cheers,

David