[rfc patch-rt] radix-tree: Partially disable memcg accounting in radix_tree_node_alloc()

[Mike Galbraith]

radix-tree: Partially disable memcg accounting in radix_tree_node_alloc()

Having no preload, which turns accounting off for non-rt kernels, trying to
allocate coming from shmem_fault() when memcg is full sends us scurrying off
to pagefault_out_of_memory(), with dramatic (usually terminal) consequences.
LTP's madvise06 testcase triggers this quite well, and per gitk, the below
was the beginning of RT memcg woes.

58e698af4c63 radix-tree: account radix_tree_node to memory cgroup

Turn memcg accounting off for RT in the problematic path.

Signed-off-by: Mike Galbraith <efault@xxxxxx>
Cc: stable-rt@xxxxxxxxxxxxxxx # +v4.6-rt
---
 lib/radix-tree.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -303,6 +303,13 @@ radix_tree_node_alloc(struct radix_tree_
 	if (!gfpflags_allow_blocking(gfp_mask) && !in_interrupt()) {
 		struct radix_tree_preload *rtp;
 
+#if defined(CONFIG_PREEMPT_RT_FULL) && defined(CONFIG_MEMCG)
+		/*
+		 * Arriving here from shmem_fault() and meeting a full memcg
+		 * will send us to pagefault_out_of_memory(), and a dead box.
+		 */
+		gfp_mask &= ~__GFP_ACCOUNT;
+#endif
 		/*
 		 * Even if the caller has preloaded, try to allocate from the
 		 * cache first for the new node to get accounted to the memory