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

[Lance Yang <lance.yang@linux.dev>]

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@nvidia.com>
>>> ---
>>>  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@nvidia.com/
>
>Cheers,
>Lance
>