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

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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)




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