• Hugh Dickins's avatar
    mm: delete unnecessary and unsafe init_tlb_ubc() · b385d21f
    Hugh Dickins authored
    init_tlb_ubc() looked unnecessary to me: tlb_ubc is statically
    initialized with zeroes in the init_task, and copied from parent to
    child while it is quiescent in arch_dup_task_struct(); so I went to
    delete it.
    
    But inserted temporary debug WARN_ONs in place of init_tlb_ubc() to
    check that it was always empty at that point, and found them firing:
    because memcg reclaim can recurse into global reclaim (when allocating
    biosets for swapout in my case), and arrive back at the init_tlb_ubc()
    in shrink_node_memcg().
    
    Resetting tlb_ubc.flush_required at that point is wrong: if the upper
    level needs a deferred TLB flush, but the lower level turns out not to,
    we miss a TLB flush.  But fortunately, that's the only part of the
    protocol that does not nest: with the initialization removed, cpumask
    collects bits from upper and lower levels, and flushes TLB when needed.
    
    Fixes: 72b252ae ("mm: send one IPI per CPU to TLB flush all entries after unmapping pages")
    Signed-off-by: default avatarHugh Dickins <hughd@google.com>
    Acked-by: default avatarMel Gorman <mgorman@techsingularity.net>
    Cc: stable@vger.kernel.org # 4.3+
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    b385d21f
vmscan.c 111 KB