RE: [PATCH] mm: implement WasActive page flag (for improvingcleancache)

[Dan Magenheimer]

> From: Andrew Morton [mailto:akpm@xxxxxxxxxxxxxxxxxxxx]
> Subject: Re: [PATCH] mm: implement WasActive page flag (for improving cleancache)

Thanks for the reply!
 
> On Thu, 26 Jan 2012 13:28:02 -0800 (PST)
> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote:
> 
> > > I do think it also needs to get cleared on the way in to the page
> > > allocator.  Otherwise:
> > >
> > > 	PageSetWasActive(page);
> > > 	free_page(page);
> > > 	...
> > > 	another_user_page = get_free_page()
> > > 	// now cleancache sees the active bit for the prev user
> > >
> > > Or am I missing somewhere it gets cleared non-explicitly somewhere?
> >
> > True, it is not getting cleared and it should be, good catch!
> 
> It should be added to PAGE_FLAGS_CHECK_AT_FREE.

I was thinking of clearing it in free_pages_prepare() before the
call to free_pages_check().  Does that make sense?  If so, then
it could also be added to PAGE_FLAGS_CHECK_AT_FREE, though it might
be a bit redundant.

> > I'll find the place to add the call to ClearPageWasActive() for v2.
> 
> AFAICT this patch consumes our second-last page flag, or close to it.
> We'll all be breaking out in hysterics when the final one is gone.

I'd be OK with only using this on 64-bit systems, though there
are ARM folks playing with zcache that might disagree.  Am I
correct in assuming that your "second-last page flag" concern
applies only to 32-bit systems?

> This does appear to be a make or break thing for cleancache - if we
> can't fix https://lkml.org/lkml/2012/1/22/61 then cleancache is pretty
> much a dead duck.

Hmmm... is that URL correct?  If so, there is some subtlety in
that thread that I am missing as I don't understand the relationship
to cleancache at all?

> But I'm going to ask for great effort to avoid
> consuming another page flag.  Either fix cleancache via other means or,
> much less desirably, find an existing page flag and overload it.

Cleancache isn't broken.  The fix is not a requirement for other
cleancache users (Xen and RAMster), though it is definitely useful.
It's not a _requirement_ for zcache either but definitely helps on
certain workloads and systems, see below.

> And I'm afraid that neither I nor other MM developers are likely to
> help you with "fix cleancache via other means" because we weren't
> provided with any description of what the problem is within cleancache,
> nor how it will be fixed.  All we are given is the assertion "cleancache
> needs this".

The patch comment says:

The patch resolves issues reported with cleancache which occur
especially during streaming workloads on older processors,
see https://lkml.org/lkml/2011/8/17/351

I can see that may not be sufficient, so let me expand on it.

First, just as page replacement worked prior to the active/inactive
redesign at 2.6.27, cleancache works without the WasActive page flag.
However, just as pre-2.6.27 page replacement had problems on
streaming workloads, so does cleancache.  The WasActive page flag
is an attempt to pass the same active/inactive info gathered by
the post-2.6.27 kernel into cleancache, with the same objectives and
presumably the same result: improving the "quality" of pages preserved
in memory thus reducing refaults.

Is that clearer?  If so, I'll do better on the description at v2.

Thanks,
Dan
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