writeback: the kupdate expire timestamp should be a moving target
Dynamically compute the dirty expire timestamp at queue_io() time. writeback_control.older_than_this used to be determined at entrance to the kupdate writeback work. This _static_ timestamp may go stale if the kupdate work runs on and on. The flusher may then stuck with some old busy inodes, never considering newly expired inodes thereafter. This has two possible problems: - It is unfair for a large dirty inode to delay (for a long time) the writeback of small dirty inodes. - As time goes by, the large and busy dirty inode may contain only _freshly_ dirtied pages. Ignoring newly expired dirty inodes risks delaying the expired dirty pages to the end of LRU lists, triggering the evil pageout(). Nevertheless this patch merely addresses part of the problem. v2: keep policy changes inside wb_writeback() and keep the wbc.older_than_this visibility as suggested by Dave. CC: Dave Chinner <david@fromorbit.com> Acked-by: NJan Kara <jack@suse.cz> Acked-by: NMel Gorman <mel@csn.ul.ie> Signed-off-by: NItaru Kitayama <kitayama@cl.bb4u.ne.jp> Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>
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