Re: 2.6.35 Regression: Ages spent discarding blocks that weren't used!

[Christoph Hellwig <hch@infradead.org>]

On Fri, Aug 06, 2010 at 03:07:25PM -0700, Hugh Dickins wrote:
> If REQ_SOFTBARRIER means that the device is still free to reorder a
> write, which was issued after discard completion was reported, before
> the discard (so later discarding the data written), then certainly I
> agree with Christoph (now Cc'ed) that the REQ_HARDBARRIER is
> unavoidable there; but if not, then it's not needed for the swap case.
>  I hope to gain a little more enlightenment on such barriers shortly.

REQ_SOFTBARRIER is indeed purely a reordering barrier inside the block
elevator.

> What does seem over the top to me, is for mm/swapfile.c's
> blkdev_issue_discard()s to be asking for both BLKDEV_IFL_WAIT and
> BLKDEV_IFL_BARRIER: those swap discards were originally written just
> to use barriers, without needing to wait for completion in there.  I'd
> be interested to hear if cutting out the BLKDEV_IFL_WAITs makes the
> swap discards behave acceptably again for you - but understand that
> you won't have a chance to try that until later next week.

That does indeed look incorrect to me.  Any kind of explicit waits
usually mean the caller provides ordering.  Getting rid of
BLKDEV_IFL_BARRIER in the swap code ASAP would indeed be beneficial
given that we are trying to get rid of hard barriers completely soon.
Auditing the existing blkdev_issue_discard callers in filesystems
is high on the todo list for me.