Re: [PATCH v5 01/14] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

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

On Sun, May 03, 2026 at 09:48:40PM -0600, Nico Pache wrote:
>
>
>On 4/29/26 9:29 AM, 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_folio_support() for FSes supporting large
>> folio with at least PMD_ORDER.
>> 
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   include/linux/pagemap.h | 26 ++++++++++++++++++++++++++
>>   mm/khugepaged.c         | 10 ++++++++--
>>   2 files changed, 34 insertions(+), 2 deletions(-)
>> 
>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>> index 1f50991b43e3b..1fed3414fe9b8 100644
>> --- a/include/linux/pagemap.h
>> +++ b/include/linux/pagemap.h
>> @@ -513,6 +513,32 @@ static inline bool mapping_large_folio_support(const struct address_space *mappi
>>   	return mapping_max_folio_order(mapping) > 0;
>>   }
>>   
>> +/**
>> + * mapping_pmd_folio_support() - Check if a mapping support PMD-sized folio
>> + * @mapping: The address_space
>> + *
>> + * Some file supports large folio but does not support as large as PMD order.
>> + * If a PMD-sized pagecache folio is attempted to be created on a filesystem,
>> + * this check needs to be performed first.
>> + *
>> + * Return: true - PMD-sized folio is supported, false - PMD-sized folio is not
>> + * supported.
>> + */
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +static inline bool mapping_pmd_folio_support(const struct address_space *mapping)
>> +{
>> +	/* AS_FOLIO_ORDER is only reasonable for pagecache folios */
>> +	VM_WARN_ON_ONCE((unsigned long)mapping & FOLIO_MAPPING_ANON);
>> +
>> +	return mapping_max_folio_order(mapping) >= PMD_ORDER;
>
>Probably a stupid question, but I dont know FS thats well.
>
>Here we are checking that the max allowed folio order is greater than 
>(or eq) to the PMD_ORDER. Yet the function asks if PMD specifically is 
>supported. In the future could we have some FS that does not support PMD 
>orders, but does support larger orders (eg. PUD)?

Good point.

IIUC, mapping_max_folio_order() means "maximum supported order" not
"the only supported order", so mapping_pmd_folio_support() just means
"PMD order is within the supported range".

Also, mapping_set_large_folios() sets the range to:

mapping_set_folio_order_range(mapping, 0, MAX_PAGECACHE_ORDER);

and __filemap_get_folio_mpol() treats max as a cap, then falls back down
towards min.

That said, if we want the helper name to mean "PMD order specifically is
supported", the more future-proof test would be:

mapping_min_folio_order(mapping) <= PMD_ORDER &&
mapping_max_folio_order(mapping) >= PMD_ORDER

Thoughs?