mm: delete unnecessary and unsafe init_tlb_ubc()
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: NHugh Dickins <hughd@google.com> Acked-by: NMel Gorman <mgorman@techsingularity.net> Cc: stable@vger.kernel.org # 4.3+ Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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