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

[Andrew Morton <akpm@osdl.org>]

On Wed, 31 Jan 2007 22:43:31 +0100
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.
> 
> Andrew, any strong opinions?

Not really.  If we change something in there, some workloads will get
better, some will get worse and most will be unaffected and any regressions
we cause won't be known until six months later.  The usual deal.

Remember that all this info is supposed to be estimating what is likely to
happen to this page in the future - we're not interested in what happened
in the past, per-se.

I'd have thought that if multiple processes are touching the same
page, this is a reason to think that the page will be required again in the
immediate future.  But you seem to think otherwise?

> NOTE - the page_mapcount(page) > 1, idea seems interesting but lets not
> go there, yet..
> 
> NOTE - recall, that in the PG_useonce patches mark_page_accessed() will
> again be a simple:
> 
>   if (!PageReferenced(page))
>     SetPageReferenced(page);
> 
> If only I could come up with a proper set of tests that covers all
> this...

Well yes, that's rather a sore point.  It's tough.  I wonder what $OTHER_OS
developers have done.  Probably their tests are priority ordered by
$market_share of their user's applications :(

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