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

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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()?

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