[RFC PATCH 00/10] Make O_SYNC writethrough

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

This is very much in development and basically untested, but Damian
started describing to me something that he wanted, and I told him he
was asking for the wrong thing, and I already had this patch series
in progress.  If someone wants to pick it up and make it mergable,
that'd be grand.

The idea is that an O_SYNC write is always going to want to write, and
we know that at the time we're storing into the page cache.  So for an
otherwise clean folio, we can skip the part where we dirty the folio,
find the dirty folios and wait for their writeback.  We can just mark the
folio as writeback-in-progress and start the IO there and then (where we
know exactly which blocks need to be written, so possibly a smaller I/O
than writing the entire page).  The existing "find dirty pages, start
I/O and wait on them" code will end up waiting on this pre-started I/O
to complete, even though it didn't start any of its own I/O.

The important part is patch 9.  Everything before it is boring prep work.
I'm in two minds about whether to keep the 'write_through' bool, or
remove it.  So feel to read patches 9+10 squashed together, or as if
patch 10 doesn't exist.  Whichever feels better.

The biggest problem with all this is that iomap doesn't have the necessary
information to cause extent allocation, so if you do an O_SYNC write
to an extent which is HOLE or DELALLOC, we can't do this optimisation.
Maybe that doesn't really matter for interesting applications.  I suspect
it doesn't matter for ZoneFS.

Matthew Wilcox (Oracle) (10):
  iomap: Pass struct iomap to iomap_alloc_ioend()
  iomap: Remove iomap_writepage_ctx from iomap_can_add_to_ioend()
  iomap: Do not pass iomap_writepage_ctx to iomap_add_to_ioend()
  iomap: Accept a NULL iomap_writepage_ctx in iomap_submit_ioend()
  iomap: Allow a NULL writeback_control argument to iomap_alloc_ioend()
  iomap: Pass a length to iomap_add_to_ioend()
  iomap: Reorder functions
  iomap: Reorder functions
  iomap: Add writethrough for O_SYNC
  remove write_through bool

 fs/iomap/buffered-io.c | 492 +++++++++++++++++++++++------------------
 1 file changed, 273 insertions(+), 219 deletions(-)

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2.34.1