Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

[Zi Yan <ziy@nvidia.com>]

On 27 Mar 2026, at 8:07, Lorenzo Stoakes (Oracle) wrote:

> On Thu, Mar 26, 2026 at 09:42:47PM -0400, Zi Yan wrote:
>> collapse_file() requires FSes supporting large folio with at least
>> PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. shmem with
>> huge option turned on also sets large folio order on mapping, so the check
>> also applies to shmem.
>>
>> While at it, replace VM_BUG_ON with returning failure values.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>
>
>
>> ---
>>  mm/khugepaged.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index d06d84219e1b..45b12ffb1550 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1899,8 +1899,11 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>>  	int nr_none = 0;
>>  	bool is_shmem = shmem_file(file);
>>
>> -	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
>> -	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
>> +	/* "huge" shmem sets mapping folio order and passes the check below */
>
> I think this isn't quite clear and could be improved to e.g.:
>
> 	/*
> 	 * Either anon shmem supports huge pages as set by shmem_enabled sysfs,
> 	 * or a shmem file system mounted with the "huge" option.
> 	 */
>
>> +	if (mapping_max_folio_order(mapping) < PMD_ORDER)
>> +		return SCAN_FAIL;
>
> As per rest of thread, this looks correct.

Will respond to that thread.

>
>> +	if (start & (HPAGE_PMD_NR - 1))
>> +		return SCAN_ADDRESS_RANGE;
>
> Hmm, we're kinda making this - presumably buggy situation - into a valid input
> that just fails the scan.
>
> Maybe just make it a VM_WARN_ON_ONCE()? Or if we want to avoid propagating the
> bug that'd cause it any further:
>
> 	if (start & (HPAGE_PMD_NR - 1)) {
> 		VM_WARN_ON_ONCE(true);
> 		return SCAN_ADDRESS_RANGE;
> 	}
>
> Or similar.

As I responded to David, will change it to VM_WARN_ON_ONCE().

>
>>
>>  	result = alloc_charge_folio(&new_folio, mm, cc);
>>  	if (result != SCAN_SUCCEED)
>> --
>> 2.43.0
>>
>
> Cheers, Lorenzo


Best Regards,
Yan, Zi