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    SubjectRe: [PATCH 1/7] mm: vmscan: Do not writeback filesystem pages in direct reclaim
    On Wed, 10 Aug 2011 11:47:14 +0100
    Mel Gorman <mgorman@suse.de> wrote:

    > From: Mel Gorman <mel@csn.ul.ie>
    >
    > 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 <mgorman@suse.de>
    > Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

    Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>



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