Re: [patch] not to disturb page LRU state when unmapping memory range

["Ken Chen" <kenchen@google.com>]

On 1/31/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Wed, 2007-01-31 at 18:02 +0000, Hugh Dickins wrote:
>
> > I'm sympathetic, but I'm going to chicken out on this one.  It was
> > me who made that set_page_dirty and mark_page_accessed conditional on
> > !PageAnon: because I didn't like the waste of time either, and could
> > see it was pointless in the PageAnon case.  But the situation is much
> > less clear to me in the file case, and it is very longstanding code.
>
> > Peter's SetPageReferenced compromise seems appealing: I'd feel better
> > about it if we had other raw uses of SetPageReferenced in the balancing
> > code, to follow as precedents.  There used to be one in do_anonymous_page,
> > but Nick and I found that an odd-one-out and conspired to have it removed
> > in 2.6.16.
>
> The trouble seems to be that mark_page_accessed() is deformed by this
> use once magick. And that really works against us in this case.
>
> The fact is that these pages can have multiple mappings triggering
> multiple calls to mark_page_accessed() launching these pages into the
> active set. Which clearly seems wrong to me.
>
> I'll go over other callers tomorrow, but I'd really like to change this
> to SetPageReferenced(), this will just preserve the PTE young state and
> let page reclaim do its usual thing.

I agree with Peter on changing it to SetPageReferenced() as a middle
ground.  Tested and it does relief majority of the problem by eliminate
calls to activate_page().  Ack'ing on Peter's earlier patch.

Acked-by: Ken Chen <kenchen@google.com>

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