On Fri, 2007-06-29 at 19:56 +0100, Hugh Dickins wrote:

I don't dare comment on your page_mkclean_one patch (5/5),
that dirty page business has grown too subtle for me.

Oh yes, the dirty handling is tricky. I had to fix a really nasty bug
with it lately. As for page_mkclean_one the difference is that it
doesn't claim a page is dirty if only the write protect bit has not been
set. If we manage to lose dirty bits from ptes and have to rely on the
write protect bit to take over the job, then we have a different problem
altogether, no ?

Your cleanups 2-4 look good, especially the mm_types.h one (how
confident are you that everything builds?), and I'm glad we can
now lay ptep_establish to rest.  Though I think you may have 
missed removing a __HAVE_ARCH_PTEP... from frv at least?

Ok, thanks for the review. I take a look at frv to see if I missed
something.

But this one...

On Fri, 29 Jun 2007, Martin Schwidefsky wrote:

quoted

If need_resched() is false it is unnecessary to call tlb_finish_mmu()
and tlb_gather_mmu() for each vma in unmap_vmas(). Moving the tlb gather
restart under the if that contains the cond_resched() will avoid
unnecessary tlb flush operations that are triggered by tlb_finish_mmu() 
and tlb_gather_mmu().

Signed-off-by: Martin Schwidefsky <redacted>

Sorry, no.  It looks reasonable, but unmap_vmas is treading a delicate
and uncomfortable line between hi-performance and lo-latency: you've
chosen to improve performance at the expense of latency.

That it true, my only concern had been performance. You likely have a
point here.

You think you're just moving the finish/gather to where they're
actually necessary; but the thing is, that per-cpu struct mmu_gather
is liable to accumulate a lot of unpreemptible work for the future
tlb_finish_mmu, particularly when anon pages are associated with swap.

Hmm, ok, so you are saying that we should do a flush at the end of each
vma.

So although there may be no need to resched right now, if we keep on
gathering more and more without flushing, we'll be very unresponsive
when a resched is needed later on.  Hence Ingo's ZAP_BLOCK_SIZE to
split it up, small when CONFIG_PREEMPT, more reasonable but still
limited when not.

Would it be acceptable to call tlb_flush_mmu instead of the
tlb_finish_mmu / tlb_gather_mmu pair if the condition around
cond_resched evaluates to false?
The background for this change is that I'm working on another patch that
will change the tlb flushing for s390 quite a bit. We won't have
anything to flush with tlb_finish_mmu because we will either flush all
tlbs with tlb_gather_mmu or each pte seperatly. The pages will always be
freed immediatly. If we are forced to restart the tlb gather then we'll
do multiple flush_tlb_mm because the information that we already flushed
everything is lost with tlb_finish_mmu.

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.


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