Re: [PATCH 7.2 v2 01/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

[Zi Yan]

On 14 Apr 2026, at 11:43, Lance Yang wrote:

> On Tue, Apr 14, 2026 at 11:37:24PM +0800, Lance Yang wrote:
>>
>> On Tue, Apr 14, 2026 at 12:29:04PM +0200, David Hildenbrand (Arm) wrote:
>>> On 4/13/26 21:20, 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.
>>>> MADV_COLLAPSE ignores shmem huge config, so exclude the check for shmem.
>>>>
>>>> While at it, replace VM_BUG_ON with VM_WARN_ON_ONCE.
>>>>
>>>> In collapse_scan_file(), add FS eligibility check to avoid redundant scans.
>>>>
>>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>>>> ---
>>>>  mm/khugepaged.c | 12 ++++++++++--
>>>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>> index b8452dbdb043..d2f0acd2dac2 100644
>>>> --- a/mm/khugepaged.c
>>>> +++ b/mm/khugepaged.c
>>>> @@ -1892,8 +1892,9 @@ 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));
>>>> +	/* MADV_COLLAPSE ignores shmem huge config, so do not check shmem */
>>>> +	VM_WARN_ON_ONCE(!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER);
>>>> +	VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
>>>>
>>>>  	result = alloc_charge_folio(&new_folio, mm, cc);
>>>>  	if (result != SCAN_SUCCEED)
>>>> @@ -2321,6 +2322,13 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>>>>  	int node = NUMA_NO_NODE;
>>>>  	enum scan_result result = SCAN_SUCCEED;
>>>>
>>>> +	/*
>>>> +	 * skip files without PMD-order folio support
>>>> +	 * do not check shmem, since MADV_COLLAPSE ignores shmem huge config
>>>> +	 */
>>>
>>> How is the !collapse path handled? Through thp_vma_allowable_order() in
>>> collapse_scan_mm_slot()?
>>>
>>> Wouldn't it be better to have that check exactly there?
>>
>> Right! Looks like patch #03[1] already does that, as David also pointed
>> out there :)
>>
>> With that in place, regular files should end up in file_thp_enabled(),
>> which checks that mapping_max_folio_order() >= PMD_ORDER.
>
> Forgot to add:
>
> thp_vma_allowable_order()
> 	-> file_thp_enabled()
> 	-> mapping_max_folio_order() check
>
>> For khugepaged, collapse_scan_mm_slot() calls thp_vma_allowable_order()
>> before entering the per-PMD scan loop, so ineligible regular file VMAs
>> should already get filtered there.
>>
>> madvise_collapse() also calls thp_vma_allowable_order() early, so it
>> should get the same filtering before reaching collapse_scan_file().
>>
>> So the extra check here looks redundant :)
>>
>> [1] https://lore.kernel.org/linux-mm/20260413192030.3275825-4-ziy@xxxxxxxxxx/
>>

Right. Will remove the extra check from collapse_scan_file().

Best Regards,
Yan, Zi