Re: [RFC PATCH 02/11] mm/khugepaged: remove READ_ONLY_THP_FOR_FS code in collapse_file()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/23/26 20:59, Zi Yan wrote:
> On 23 Mar 2026, at 15:53, David Hildenbrand (Arm) wrote:
> 
>> On 3/23/26 20:47, Zi Yan wrote:
>>>
>>>
>>> My understanding is that collapse_file() is only used for
>>> READ_ONLY_THP_FOR_FS. If FSes with large folio support also use it,
>>> I can replace IS_ENABLE with mapping_large_folio_support().
>>
>> Otherwise we'd be losing support for THP collapse in files? We'd have to
>> cross fingers that readahead gives us some.
>>
>> So we have to be a bit careful here. We want khugepaged to collapse THPs
>> in filesystems that support large folios even without READ_ONLY_THP_FOR_FS.
> 
> Based on[1], collapse_file() only works on read-only fd. So it is not
> as general as all files. If we do what you suggested, the function
> will be collapse read-only folios from FSes with large folio support.

Right, I think that's the first step to keep the existing use case
working. Long term, I suspect we might want to extend that to writable
files etc.

> 
> BTW, has anyone used/relied on collapse_file() on any FS with large folio
> support?

I assume so, because for executables you would have to be lucky to get a
PMD THP? So I don't see the non-khugepaged large folio support on par
with khugepaged support.

-- 
Cheers,

David