On Mon, 18 Dec 2006, Linus Torvalds wrote:
> On Tue, 19 Dec 2006, Nick Piggin wrote:
> >
> > We never want to drop dirty data! (ignoring the truncate case, which is
> > handled privately by truncate anyway)
>
> Bzzt.
>
> SURE we do.
>
> We absolutely do want to drop dirty data in the writeout path.
>
> How do you think dirty data ever _becomes_ clean data?
>
> In other words, yes, we _do_ want to test-and-clear all the pgtable bits
> _and_ the PG_dirty bit. We want to do it for:
> - writeout
> - truncate
> - possibly a "drop" event (which could be a case for a journal entry that
> becomes stale due to being replaced or something - kind of "truncate"
> on metadata)
>
> because both of those events _literally_ turn dirty state into clean
> state.
>
> In no other circumstance do we ever want to clear a dirty bit, as far as I
> can tell.
i admit this may not be entirely relevant, but it seems like a good place
to bring up an old problem: when a disk dies with lots of queued writes
it can totally bring a system to its knees... even after the disk is
removed. i wrote up something about this a while ago:
http://lkml.org/lkml/2005/8/18/243
so there's another reason to "clear a dirty bit"... well, in fact -- drop
the pages entirely.
-dean
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