On Wed, Feb 28, 2024 at 03:00:36AM +0000, Matthew Wilcox wrote: > On Tue, Feb 27, 2024 at 09:22:26PM -0500, Kent Overstreet wrote: > > Which does raise the question of if we've ever attempted to define a > > lock ordering on folios. I suspect not, since folio lock doesn't even > > seem to have lockdep support. > > We even wrote it down! > > /* > * To avoid deadlocks between range_cyclic writeback and callers > * that hold pages in PageWriteback to aggregate I/O until > * the writeback iteration finishes, we do not loop back to the > * start of the file. Doing so causes a page lock/page > * writeback access order inversion - we should only ever lock > * multiple pages in ascending page->index order, and looping > * back to the start of the file violates that rule and causes > * deadlocks. > */ > > (I'll take the AR to put this somewhere better like in the folio_lock() > kernel-doc) Um. I already did. * Context: May sleep. If you need to acquire the locks of two or * more folios, they must be in order of ascending index, if they are * in the same address_space. If they are in different address_spaces, * acquire the lock of the folio which belongs to the address_space which * has the lowest address in memory first. Where should I have put this information that you would have found it, if not in the kernel-doc for folio_lock()?
