Re: mm: dirty page problem

From: Peter Zijlstra
Date: Tue Jun 23 2009 - 09:38:29 EST

On Tue, 2009-06-23 at 21:32 +0800, xue yong wrote:
> On Tue, Jun 23, 2009 at 7:52 PM, Peter Zijlstra<peterz@xxxxxxxxxxxxx> wrote:
> > On Tue, 2009-06-23 at 19:43 +0800, xue yong wrote:
> >> Thanks a lot, Peter.
> >> Your reply resolved my doubt.
> >>
> >> we have a service program (just say A) running with about 14G mmaped data.
> >> and there is another daemon (just say B) do msync( SYNC) periodically.
> >>
> >> so I want to know in this pattern, was the data flushed to disk?
> >
> > I don't think so.
> >
> > The problem is that msync() only scans the current process' page tables,
> > which would be clean since B doesn't write, only A does.
> >
> > So you'd have to modify your program, A, to do the msync() itself --
> > possibly from a thread (threads share the vm context and thus page
> > tables).
> >
> :) I did have this thought, because there was littile bo(block out),
> and pmap showed that
> the dirty pages belong to a process was always growing.
> I believe you are the authority. Your confirmation matters.

I'm one of the people who knows this code rather well, yes ;-)

> In "Understanding the Linux Virtual Memory Manager" page 163, Mel
> Gorman said that
> Process-mapped pages are not easily swappable because there is no
> way to map struct pages to PTEs except to search every page table, which is far
> too expensive.
> So neither kswapd nor other kernel daemons do the scan job.
> Without explicit action these pages would stay hidden.

While a great book to learn some of the basics from, it is severely
out-dated. I think in his 2.6 chapter he does mention something about
reverse map, or rmap as its called.

These days we do keep a data structure whereby it is easier to find all
ptes for a particular mapping (mm/rmap.c).

In particular try_to_unmap() is the routine used to remove all ptes in
order to swap a page.

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