Re: [patch] mm, compaction: avoid isolating pinned pages fix

From: Joonsoo Kim
Date: Mon Feb 03 2014 - 20:53:43 EST

On Mon, Feb 03, 2014 at 05:20:46PM -0800, David Rientjes wrote:
> On Tue, 4 Feb 2014, Joonsoo Kim wrote:
> > I think that you need more code to skip this type of page correctly.
> > Without page_mapped() check, this code makes migratable pages be skipped,
> > since if page_mapped() case, page_count() may be more than zero.
> >
> > So I think that you need following change.
> >
> > (!page_mapping(page) && !page_mapped(page) && page_count(page))
> >
> These pages returned by get_user_pages() will have a mapcount of 1 so this
> wouldn't actually fix the massive lock contention. page_mapping() is only
> going to be NULL for pages off the lru like these are for

Okay. It can't fix your situation. Anyway, *normal* anon pages may be mapped
and have positive page_count(), so your code such as
'!page_mapping(page) && page_count(page)' makes compaction skip these *normal*
anon pages and this is incorrect behaviour.

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