• Mel Gorman's avatar
    mm, page_alloc: distinguish between being unable to sleep, unwilling to sleep... · d0164adc
    Mel Gorman authored
    mm, page_alloc: distinguish between being unable to sleep, unwilling to sleep and avoiding waking kswapd
    __GFP_WAIT has been used to identify atomic context in callers that hold
    spinlocks or are in interrupts.  They are expected to be high priority and
    have access one of two watermarks lower than "min" which can be referred
    to as the "atomic reserve".  __GFP_HIGH users get access to the first
    lower watermark and can be called the "high priority reserve".
    Over time, callers had a requirement to not block when fallback options
    were available.  Some have abused __GFP_WAIT leading to a situation where
    an optimisitic allocation with a fallback option can access atomic
    This patch uses __GFP_ATOMIC to identify callers that are truely atomic,
    cannot sleep and have no alternative.  High priority users continue to use
    __GFP_HIGH.  __GFP_DIRECT_RECLAIM identifies callers that can sleep and
    are willing to enter direct reclaim.  __GFP_KSWAPD_RECLAIM to identify
    callers that want to wake kswapd for background reclaim.  __GFP_WAIT is
    redefined as a caller that is willing to enter direct reclaim and wake
    kswapd for background reclaim.
    This patch then converts a number of sites
    o __GFP_ATOMIC is used by callers that are high priority and have memory
      pools for those requests. GFP_ATOMIC uses this flag.
    o Callers that have a limited mempool to guarantee forward progress clear
      __GFP_DIRECT_RECLAIM but keep __GFP_KSWAPD_RECLAIM. bio allocations fall
      into this category where kswapd will still be woken but atomic reserves
      are not used as there is a one-entry mempool to guarantee progress.
    o Callers that are checking if they are non-blocking should use the
      helper gfpflags_allow_blocking() where possible. This is because
      checking for __GFP_WAIT as was done historically now can trigger false
      positives. Some exceptions like dm-crypt.c exist where the code intent
      is clearer if __GFP_DIRECT_RECLAIM is used instead of the helper due to
      flag manipulations.
    o Callers that built their own GFP flags instead of starting with GFP_KERNEL
      and friends now also need to specify __GFP_KSWAPD_RECLAIM.
    The first key hazard to watch out for is callers that removed __GFP_WAIT
    and was depending on access to atomic reserves for inconspicuous reasons.
    In some cases it may be appropriate for them to use __GFP_HIGH.
    The second key hazard is callers that assembled their own combination of
    GFP flags instead of starting with something like GFP_KERNEL.  They may
    now wish to specify __GFP_KSWAPD_RECLAIM.  It's almost certainly harmless
    if it's missed in most cases as other activity will wake kswapd.
    Signed-off-by: default avatarMel Gorman <mgorman@techsingularity.net>
    Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Acked-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Vitaly Wool <vitalywool@gmail.com>
    Cc: Rik van Riel <riel@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>