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
