Re: [LSF/MM/BPF TOPIC] Measuring limits and enhancing buffered IO

Kent Overstreet
2024-02-28T17:34:52Z

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...