Re: [PATCH 7.2 v3 01/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

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

On 2026/4/23 10:51, Zi Yan wrote:
> On 22 Apr 2026, at 22:43, Lance Yang wrote:
> 
>> On Fri, Apr 17, 2026 at 10:44:18PM -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.
>>> MADV_COLLAPSE ignores shmem huge config, so exclude the check for shmem.
>>>
>>> While at it, replace VM_BUG_ON with VM_WARN_ON_ONCE.
>>>
>>> Add a helper function mapping_pmd_thp_support() for FSes supporting large
>>> folio with at least PMD_ORDER.
>>>
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> ---
>>> include/linux/pagemap.h | 10 ++++++++++
>>> mm/khugepaged.c         |  5 +++--
>>> 2 files changed, 13 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>>> index ec442af3f886..c3cb1ec982cd 100644
>>> --- a/include/linux/pagemap.h
>>> +++ b/include/linux/pagemap.h
>>> @@ -524,6 +524,16 @@ static inline bool mapping_large_folio_support(const struct address_space *mappi
>>> 	return mapping_max_folio_order(mapping) > 0;
>>> }
>>>
>>> +static inline bool mapping_pmd_thp_support(const struct address_space *mapping)
>>> +{
>>> +	/* AS_FOLIO_ORDER is only reasonable for pagecache folios */
>>> +	VM_WARN_ONCE((unsigned long)mapping & FOLIO_MAPPING_ANON,
>>> +			"Anonymous mapping always supports PMD THP");
>>
>> Nit: afraid not, at least when running on architectures without PMD leaf
>> entries ...
>>
>> Maybe better to say this helper is only meaningful for pagecache-backed
>> mappings. Anonymous mappings should not reach here.
> 
> Good suggestion. Will fix it.
> 
>>
>>> +
>>> +	return mapping_max_folio_order(mapping) >= PMD_ORDER;
>>> +}
>>> +
>>> +
>>> /* Return the maximum folio size for this pagecache mapping, in bytes. */
>>> static inline size_t mapping_max_folio_size(const struct address_space *mapping)
>>> {
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index b8452dbdb043..3eb5d982d3d3 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_pmd_thp_support(mapping));
>>
>> With [1], can we drop !is_shmem here as well? shmem would then always
>> call mapping_set_large_folios(inode->i_mapping):
>>
>> ---8<---
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 4ecefe02881d..dafbea53b22d 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3087,10 +3087,7 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
>>   	cache_no_acl(inode);
>>   	if (sbinfo->noswap)
>>   		mapping_set_unevictable(inode->i_mapping);
>> -
>> -	/* Don't consider 'deny' for emergencies and 'force' for testing */
>> -	if (sbinfo->huge)
>> -		mapping_set_large_folios(inode->i_mapping);
>> +	mapping_set_large_folios(inode->i_mapping);
>>
>>   	switch (mode & S_IFMT) {
>>   	default:
>> --
>>
>> But we can do that in a follow-up, once the revert lands :)
> 
> Right. That would make this patchset depend on Baolin’s fix. A follow-up
> patch is easier for managing these patches. I will add a TODO in the
> comment, so that we will not forget. Thank you for the suggestion.

OK. With the changes above,

Reviewed-by: Lance Yang <lance.yang@linux.dev>