On Mon, Feb 20, 2023 at 06:48:38PM -0800, Hugh Dickins wrote: > Yes, that's a good principle, that we should avoid to lock/wait > synchronously once we have locked one folio (hmm, above you say > "more than one": I think we mean the same thing, we're just > stating it differently, given how the code runs at present). I suspect the migrate page code is disobeying the locking ordering rules for multiple folios. if two folios belong to the same file, they must be locked by folio->index order, low to high. If two folios belong to different files, they must be ordered by folio->mapping, the mapping lowest in memory first. You can see this locking rule embedded in vfs_lock_two_folios() in fs/remap_range.c. I don't know what the locking rules are for two folios which are not file folios, or for two folios when one is anonymous and the other belongs to a file. Maybe it's the same; you can lock them ordered by ->mapping first, then by ->index. Or you can just trylock multiple folios and skip the ones that don't work.
