On Thu, Sep 22, 2016 at 10:41:50AM -0700, Hugh Dickins wrote:

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: 72b252aed506 ("mm: send one IPI per CPU to TLB flush all entries after unmapping pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # 4.3+

Acked-by: Mel Gorman <redacted>

-- 
Mel Gorman
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>