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    mm/page-writeback.c: fix dirty_balance_reserve subtraction from dirtyable memory · a804552b
    Johannes Weiner 提交于
    Tejun reported stuttering and latency spikes on a system where random
    tasks would enter direct reclaim and get stuck on dirty pages.  Around
    50% of memory was occupied by tmpfs backed by an SSD, and another disk
    (rotating) was reading and writing at max speed to shrink a partition.
    
    : The problem was pretty ridiculous.  It's a 8gig machine w/ one ssd and 10k
    : rpm harddrive and I could reliably reproduce constant stuttering every
    : several seconds for as long as buffered IO was going on on the hard drive
    : either with tmpfs occupying somewhere above 4gig or a test program which
    : allocates about the same amount of anon memory.  Although swap usage was
    : zero, turning off swap also made the problem go away too.
    :
    : The trigger conditions seem quite plausible - high anon memory usage w/
    : heavy buffered IO and swap configured - and it's highly likely that this
    : is happening in the wild too.  (this can happen with copying large files
    : to usb sticks too, right?)
    
    This patch (of 2):
    
    The dirty_balance_reserve is an approximation of the fraction of free
    pages that the page allocator does not make available for page cache
    allocations.  As a result, it has to be taken into account when
    calculating the amount of "dirtyable memory", the baseline to which
    dirty_background_ratio and dirty_ratio are applied.
    
    However, currently the reserve is subtracted from the sum of free and
    reclaimable pages, which is non-sensical and leads to erroneous results
    when the system is dominated by unreclaimable pages and the
    dirty_balance_reserve is bigger than free+reclaimable.  In that case, at
    least the already allocated cache should be considered dirtyable.
    
    Fix the calculation by subtracting the reserve from the amount of free
    pages, then adding the reclaimable pages on top.
    
    [akpm@linux-foundation.org: fix CONFIG_HIGHMEM build]
    Signed-off-by: NJohannes Weiner <hannes@cmpxchg.org>
    Reported-by: NTejun Heo <tj@kernel.org>
    Tested-by: NTejun Heo <tj@kernel.org>
    Reviewed-by: NRik van Riel <riel@redhat.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Wu Fengguang <fengguang.wu@intel.com>
    Reviewed-by: NMichal Hocko <mhocko@suse.cz>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
    a804552b
page-writeback.c 75.1 KB