Re: [PATCH v5 06/19] selftests/mm: stop khugepaged during the MADV_COLLAPSE cases

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/9/26 6:41 PM, Kiryl Shutsemau wrote:
> On Wed, Sep 09, 2026 at 05:55:01PM +0800, Baolin Wang wrote:
>>
>>
>> On 9/8/26 8:50 PM, Kiryl Shutsemau wrote:
>>> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>>>
>>> __madvise_collapse() turns THP off before each MADV_COLLAPSE, both to keep
>>> khugepaged out of the range and to prove MADV_COLLAPSE ignores the setting.
>>> It clears the global controls only, which is no longer enough.  A per-order
>>> control overrides them, and -s, which makes the cases fault in folios of
>>> one order, leaves that order's control at "always".  khugepaged then
>>> collapses the very range the case is working on, and the case fails on a
>>> collapse that was interfered with rather than refused.
>>
>> Right. So I think the correct fix tag is b7f16963efe7 ("mm/khugepaged: run
>> khugepaged for all orders"), because before this commit, khugepaged would
>> not try to collapse this range since it only checked whether the PMD order
>> was suitable for collapse.
> 
> Agreed.  The series is in mm-new already; if a respin is needed I will use
> that tag.
> 
>>> @@ -547,9 +547,16 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
>>>    	/*
>>>    	 * Prevent khugepaged interference and tests that MADV_COLLAPSE
>>>    	 * ignores /sys/kernel/mm/transparent_hugepage/enabled
>>> +	 *
>>> +	 * "inherit" rather than "never" so that MADV_COLLAPSE on shmem still
>>> +	 * finds an order to build.
>>>    	 */
>>>    	settings.thp_enabled = THP_NEVER;
>>>    	settings.shmem_enabled = SHMEM_NEVER;
>>> +	for (i = 0; i < NR_ORDERS; i++) {
>>> +		settings.hugepages[i].enabled = THP_INHERIT;
>>> +		settings.shmem_hugepages[i].enabled = SHMEM_INHERIT;
>>> +	}
>>
>> This looks like a workaround to me. Shouldn't we fix this in shmem instead?
> 
> Good point.
> 
> It can be a follow-up patch.  Do you want to make a proper shmem.c fix
> and update the selftest along with it?

Either way works for me.