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    SubjectRe: [PATCH 1/6] writeback: take account of NR_WRITEBACK_TEMP in balance_dirty_pages()
    On Tue, Jul 13, 2010 at 04:58:47PM +0800, Miklos Szeredi wrote:
    > 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.

    Ah OK, let's remove this patch.

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

    Right.

    > 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.

    clip_bdi_dirty_limit() is removed in the next patch, hopefully it's OK.

    > > 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.

    Right. nfs_write_inode() normally tries to commit unstable pages.

    Thanks,
    Fengguang


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