Reordering requirements

What am I missing?  Jon Corbet's article and seanyoung's implementation and possibly willy's
comment above suggest that these trim/punch/discard/... requests are somehow very special in
their reordering requirements, whereas it seems to me that a discard is just the same as a
write: the existing code mustn't reorder any writes to a given block, and the new code mustn't
reorder any write-or-discard requests to a given block.  This shouldn't require any more
barriers or speed penalty than we already have for writes.

(The only difference is that one might use *looser* requirements for reordering discards and
reads for a given block, depending on the semantics of reading a discarded block.)