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    SubjectRe: [PATCH 1/6] writeback: take account of NR_WRITEBACK_TEMP in balance_dirty_pages()
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    On Mon, 12 Jul 2010, Andrew Morton wrote:
    > On Sun, 11 Jul 2010 10:06:57 +0800
    > Wu Fengguang <fengguang.wu@intel.com> wrote:
    >
    > >
    > > Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
    > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    > > ---
    > > mm/page-writeback.c | 7 ++++---
    > > 1 file changed, 4 insertions(+), 3 deletions(-)
    > >
    > > --- linux-next.orig/mm/page-writeback.c 2010-07-11 08:41:37.000000000 +0800
    > > +++ linux-next/mm/page-writeback.c 2010-07-11 08:42:14.000000000 +0800
    > > @@ -503,11 +503,12 @@ static void balance_dirty_pages(struct a
    > > };
    > >
    > > get_dirty_limits(&background_thresh, &dirty_thresh,
    > > - &bdi_thresh, bdi);
    > > + &bdi_thresh, bdi);
    > >
    > > nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
    > > - global_page_state(NR_UNSTABLE_NFS);
    > > - nr_writeback = global_page_state(NR_WRITEBACK);
    > > + global_page_state(NR_UNSTABLE_NFS);
    > > + nr_writeback = global_page_state(NR_WRITEBACK) +
    > > + global_page_state(NR_WRITEBACK_TEMP);
    > >
    > > bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
    > > bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
    > >
    >
    > hm, OK.

    Hm, hm. I'm not sure this is right. The VM has absolutely no control
    over NR_WRITEBACK_TEMP pages, they may clear quickly or may not make
    any progress. So it's usually wrong to make a decision based on
    NR_WRITEBACK_TEMP for an unrelated device.

    Using it in throttle_vm_writeout() would actually be deadlocky, since
    the userspace filesystem will probably depend on memory allocations to
    complete the writeout.

    The only place where we should be taking NR_WRITEBACK_TEMP into
    account is calculating the remaining memory that can be devided
    between dirtyers, and that's (clip_bdi_dirty_limit) where it is
    already used.

    > I wonder whether we could/should have unified NR_WRITEBACK_TEMP and
    > NR_UNSTABLE_NFS. Their "meanings" aren't quite the same, but perhaps
    > some "treat page as dirty because the fs is futzing with it" thing.

    AFAICS NR_UNSTABLE_NFS is something akin to NR_DIRTY, only on the
    server side. So nfs can very much do something about making
    NR_UNSTABLE_NFS go away, while there's nothing that can be done about
    NR_WRITEBACK_TEMP.

    Thanks,
    Miklos


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