Re: page fault scalability patch V16 [3/4]: Drop page_table_lock

Nick Piggin
2005-02-02T01:41:38Z

On Tue, 2005-02-01 at 17:20 -0800, Christoph Lameter wrote:
> On Wed, 2 Feb 2005, Nick Piggin wrote:
>
> > > The unmapping in rmap.c would change the pte. This would be discovered
> > > after acquiring the spinlock later in do_wp_page. Which would then lead to
> > > the operation being abandoned.
> > Oh yes, but suppose your page_cache_get is happening at the same time
> > as free_pages_check, after the page gets freed by the scanner? I can't
> > actually think of anything that would cause a real problem (ie. not a
> > debug check), off the top of my head. But can you say there _isn't_
> > anything?
> >
> > Regardless, it seems pretty dirty to me. But could possibly be made
> > workable, of course.
>
> Would it make you feel better if we would move the spin_unlock back to the
> prior position? This would ensure that the fallback case is exactly the
> same.
>

Well yeah, but the interesting case is when that isn't a lock ;)

I'm not saying what you've got is no good. I'm sure it would be fine
for testing. And if it happens that we can do the "page_count doesn't
mean anything after it has reached zero and been freed. Nor will it
necessarily be zero when a new page is allocated" thing without many
problems, then this may be a fine way to do it.

I was just pointing out this could be a problem without putting a
lot of thought into it...

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