On Wed, Feb 28, 2024 at 04:22:32AM +0000, Matthew Wilcox wrote: > 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()? I should have seen that :) But even better would be if we could get lockdep to support folio locks, then we could define a lockdep comparison function. Of course, there's no place to stick a lockdep map, but I think technically lockdep could do everything it needs to do without state in the lock itself, if only that code didn't make my eyes bleed whenever I have to dig into it...
