• Roman Gushchin's avatar
    dcache: account external names as indirectly reclaimable memory · 6d794237
    Roman Gushchin authored
    commit f1782c9b upstream.
    
    I received a report about suspicious growth of unreclaimable slabs on
    some machines.  I've found that it happens on machines with low memory
    pressure, and these unreclaimable slabs are external names attached to
    dentries.
    
    External names are allocated using generic kmalloc() function, so they
    are accounted as unreclaimable.  But they are held by dentries, which
    are reclaimable, and they will be reclaimed under the memory pressure.
    
    In particular, this breaks MemAvailable calculation, as it doesn't take
    unreclaimable slabs into account.  This leads to a silly situation, when
    a machine is almost idle, has no memory pressure and therefore has a big
    dentry cache.  And the resulting MemAvailable is too low to start a new
    workload.
    
    To address the issue, the NR_INDIRECTLY_RECLAIMABLE_BYTES counter is
    used to track the amount of memory, consumed by external names.  The
    counter is increased in the dentry allocation path, if an external name
    structure is allocated; and it's decreased in the dentry freeing path.
    
    To reproduce the problem I've used the following Python script:
    
      import os
    
      for iter in range (0, 10000000):
          try:
              name = ("/some_long_name_%d" % iter) + "_" * 220
              os.stat(name)
          except Exception:
              pass
    
    Without this patch:
      $ cat /proc/meminfo | grep MemAvailable
      MemAvailable:    7811688 kB
      $ python indirect.py
      $ cat /proc/meminfo | grep MemAvailable
      MemAvailable:    2753052 kB
    
    With the patch:
      $ cat /proc/meminfo | grep MemAvailable
      MemAvailable:    7809516 kB
      $ python indirect.py
      $ cat /proc/meminfo | grep MemAvailable
      MemAvailable:    7749144 kB
    
    [guro@fb.com: fix indirectly reclaimable memory accounting for CONFIG_SLOB]
      Link: http://lkml.kernel.org/r/20180312194140.19517-1-guro@fb.com
    [guro@fb.com: fix indirectly reclaimable memory accounting]
      Link: http://lkml.kernel.org/r/20180313125701.7955-1-guro@fb.com
    Link: http://lkml.kernel.org/r/20180305133743.12746-5-guro@fb.comSigned-off-by: default avatarRoman Gushchin <guro@fb.com>
    Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Mel Gorman <mgorman@techsingularity.net>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    6d794237