• Mel Gorman's avatar
    mm: vmscan: block kswapd if it is encountering pages under writeback · 283aba9f
    Mel Gorman authored
    Historically, kswapd used to congestion_wait() at higher priorities if
    it was not making forward progress.  This made no sense as the failure
    to make progress could be completely independent of IO.  It was later
    replaced by wait_iff_congested() and removed entirely by commit 258401a6
    (mm: don't wait on congested zones in balance_pgdat()) as it was
    duplicating logic in shrink_inactive_list().
    
    This is problematic.  If kswapd encounters many pages under writeback
    and it continues to scan until it reaches the high watermark then it
    will quickly skip over the pages under writeback and reclaim clean young
    pages or push applications out to swap.
    
    The use of wait_iff_congested() is not suited to kswapd as it will only
    stall if the underlying BDI is really congested or a direct reclaimer
    was unable to write to the underlying BDI.  kswapd bypasses the BDI
    congestion as it sets PF_SWAPWRITE but even if this was taken into
    account then it would cause direct reclaimers to stall on writeback
    which is not desirable.
    
    This patch sets a ZONE_WRITEBACK flag if direct reclaim or kswapd is
    encountering too many pages under writeback.  If this flag is set and
    kswapd encounters a PageReclaim page under writeback then it'll assume
    that the LRU lists are being recycled too quickly before IO can complete
    and block waiting for some IO to complete.
    Signed-off-by: default avatarMel Gorman <mgorman@suse.de>
    Reviewed-by: default avatarMichal Hocko <mhocko@suse.cz>
    Acked-by: default avatarRik van Riel <riel@redhat.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Jiri Slaby <jslaby@suse.cz>
    Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
    Tested-by: default avatarZlatko Calusic <zcalusic@bitsync.net>
    Cc: dormando <dormando@rydia.net>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    283aba9f
vmscan.c 104 KB