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    SubjectRe: [PATCH 1/8] mm: vmscan: Do not writeback filesystem pages in direct reclaim
    On Mon, Aug 01, 2011 at 12:06:06AM +0900, Minchan Kim wrote:
    > On Thu, Jul 21, 2011 at 05:28:43PM +0100, Mel Gorman wrote:
    > > From: Mel Gorman <>
    > >
    > > When kswapd is failing to keep zones above the min watermark, a process
    > > will enter direct reclaim in the same manner kswapd does. If a dirty
    > > page is encountered during the scan, this page is written to backing
    > > storage using mapping->writepage.
    > >
    > > This causes two problems. First, it can result in very deep call
    > > stacks, particularly if the target storage or filesystem are complex.
    > > Some filesystems ignore write requests from direct reclaim as a result.
    > > The second is that a single-page flush is inefficient in terms of IO.
    > > While there is an expectation that the elevator will merge requests,
    > > this does not always happen. Quoting Christoph Hellwig;
    > >
    > > The elevator has a relatively small window it can operate on,
    > > and can never fix up a bad large scale writeback pattern.
    > >
    > > This patch prevents direct reclaim writing back filesystem pages by
    > > checking if current is kswapd. Anonymous pages are still written to
    > > swap as there is not the equivalent of a flusher thread for anonymous
    > > pages. If the dirty pages cannot be written back, they are placed
    > > back on the LRU lists. There is now a direct dependency on dirty page
    > > balancing to prevent too many pages in the system being dirtied which
    > > would prevent reclaim making forward progress.
    > >
    > > Signed-off-by: Mel Gorman <>
    > Reviewed-by: Minchan Kim <>


    > Nitpick.
    > We can change description of should_reclaim_stall.
    > "Returns true if the caller should wait to clean dirty/writeback pages"
    > ->
    > "Returns true if direct reclaimer should wait to clean writeback pages"

    Not a nitpick. At least one check for RECLAIM_MODE_SYNC is no longer
    reachable. I've added a new patch that updates the comment and has
    synchronous direct reclaim wait on pages under writeback.

    Mel Gorman
    SUSE Labs

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