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    SubjectRe: [RFC 1/2] deactive invalidated pages
    On Tue, Nov 23, 2010 at 12:35:35PM -0800, Andrew Morton wrote:
    > On Tue, 23 Nov 2010 14:58:56 +0000
    > Mel Gorman <mel@csn.ul.ie> wrote:
    >
    > > On Tue, Nov 23, 2010 at 09:55:49AM -0500, Ben Gamari wrote:
    > > > On Tue, 23 Nov 2010 09:38:59 +0000, Mel Gorman <mel@csn.ul.ie> wrote:
    > > > > > If it's mapped pagecache then the user was being a bit silly (or didn't
    > > > > > know that some other process had mapped the file). In which case we
    > > > > > need to decide what to do - leave the page alone, deactivate it, or
    > > > > > half-deactivate it as this patch does.
    > > > > >
    > > > >
    > > > > What are the odds of an fadvise() user having used mincore() in advance
    > > > > to determine if the page was in use by another process? I would guess
    > > > > "low" so this half-deactivate gives a chance for the page to be promoted
    > > > > again as well as a chance for the flusher threads to clean the page if
    > > > > it really is to be reclaimed.
    > > > >
    > > > Do we really want to make the user jump through such hoops as using
    > > > mincore() just to get the kernel to handle use-once pages properly?
    > >
    > > I would think "no" which is why I support half-deactivating pages so they won't
    > > have to.
    >
    > If the page is page_mapped() then we can assume that some other process
    > is using it and we leave it alone *altogether*.
    >

    Agreed, that makes perfect sense.

    > If the page is dirty or under writeback (and !page_mapped()) then we
    > should assume that we should free it asap. The PageReclaim() trick
    > might help with that.
    >

    Again agreed.

    > I just don't see any argument for moving the page to the head of the
    > inactive LRU as a matter of policy. We can park it there because we
    > can't think of anythnig else to do with it, but it's the wrong place
    > for it.
    >

    Is there a better alternative? One thing that springs to mind is that we are
    not exactly tracking very well what effect these policy changes have. The
    analysis scripts I have do a reasonable job on tracking reclaim activity
    (although only as part of the mmtests tarball, I should split them out as
    a standalone tool) but not the impact - namely minor and major faults. I
    should sort that out so we can put better reclaim analysis in place.

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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