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    SubjectRe: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim
    On Wed, 16 Jun 2010 10:40:36 +0900
    KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

    > On Tue, 15 Jun 2010 20:53:43 -0400
    > Rik van Riel <riel@redhat.com> wrote:
    >
    > > On 06/15/2010 08:39 PM, KAMEZAWA Hiroyuki wrote:
    > >
    > > > Hmm, or do you recommend no-dirty-page-writeback when a memcg hits limit ?
    > > > Maybe we'll see much swaps.
    > > >
    > > > I want to go with this for a while, changing memcg's behavior will took
    > > > some amounts of time, there are only a few developpers.
    > >
    > > One thing we can do, for kswapd, memcg and direct reclaim alike,
    > > is to tell the flusher threads to flush pages related to a pageout
    > > candidate page to disk.
    > >
    > > That way the reclaiming processes can wait on some disk IO to
    > > finish, while the flusher thread takes care of the actual flushing.
    > >
    > > That should also fix the "kswapd filesystem IO has really poor IO
    > > patterns" issue.
    > >
    > > There's no reason not to fix this issue the right way.
    > >
    > yes. but this patch just stops writeback. I think it's sane to ask
    > not to change behavior until there are some useful changes in flusher
    > threads.
    >
    > IMO, until flusher threads can work with I/O cgroup, memcg shoudln't
    > depend on it because writeback allows stealing resource without it.
    >

    BTW, copy_from_user/copy_to_user is _real_ problem, I'm afraid following
    much more than memcg.

    handle_mm_fault()
    -> handle_pte_fault()
    -> do_wp_page()
    -> balance_dirty_page_rate_limited()
    -> balance_dirty_pages()
    -> writeback_inodes_wbc()
    -> writeback_inodes_wb()
    -> writeback_sb_inodes()
    -> writeback_single_inode()
    -> do_writepages()
    -> generic_write_pages()
    -> write_cache_pages() // use on-stack pagevec.
    -> writepage()

    maybe much more stack consuming than memcg->writeback after vmscan.c diet.

    Bye.
    -Kame




















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