[PATCH v2 1/2] mm/cgroup: avoid panic when init with low memory

[Laurent Dufour <ldufour@linux.vnet.ibm.com>]

The system may panic when initialisation is done when almost all the
memory is assigned to the huge pages using the kernel command line
parameter hugepage=xxxx. Panic may occur like this:

[    0.082289] Unable to handle kernel paging request for data at address 0x00000000
[    0.082338] Faulting instruction address: 0xc000000000302b88
[    0.082377] Oops: Kernel access of bad area, sig: 11 [#1]
[    0.082408] SMP NR_CPUS=2048 [    0.082424] NUMA
[    0.082440] pSeries
[    0.082457] Modules linked in:
[    0.082490] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.9.0-15-generic #16-Ubuntu
[    0.082536] task: c00000021ed01600 task.stack: c00000010d108000
[    0.082575] NIP: c000000000302b88 LR: c000000000270e04 CTR: c00000000016cfd0
[    0.082621] REGS: c00000010d10b2c0 TRAP: 0300   Not tainted (4.9.0-15-generic)
[    0.082666] MSR: 8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>[ 0.082770]   CR: 28424422  XER: 00000000
[    0.082793] CFAR: c0000000003d28b8 DAR: 0000000000000000 DSISR: 40000000 SOFTE: 1
GPR00: c000000000270e04 c00000010d10b540 c00000000141a300 c00000010fff6300
GPR04: 0000000000000000 00000000026012c0 c00000010d10b630 0000000487ab0000
GPR08: 000000010ee90000 c000000001454fd8 0000000000000000 0000000000000000
GPR12: 0000000000004400 c00000000fb80000 00000000026012c0 00000000026012c0
GPR16: 00000000026012c0 0000000000000000 0000000000000000 0000000000000002
GPR20: 000000000000000c 0000000000000000 0000000000000000 00000000024200c0
GPR24: c0000000016eef48 0000000000000000 c00000010fff7d00 00000000026012c0
GPR28: 0000000000000000 c00000010fff7d00 c00000010fff6300 c00000010d10b6d0
NIP [c000000000302b88] mem_cgroup_soft_limit_reclaim+0xf8/0x4f0
[    0.083456] LR [c000000000270e04] do_try_to_free_pages+0x1b4/0x450
[    0.083494] Call Trace:
[    0.083511] [c00000010d10b540] [c00000010d10b640] 0xc00000010d10b640 (unreliable)
[    0.083567] [c00000010d10b610] [c000000000270e04] do_try_to_free_pages+0x1b4/0x450
[    0.083622] [c00000010d10b6b0] [c000000000271198] try_to_free_pages+0xf8/0x270
[    0.083676] [c00000010d10b740] [c000000000259dd8] __alloc_pages_nodemask+0x7a8/0xff0
[    0.083729] [c00000010d10b960] [c0000000002dd274] new_slab+0x104/0x8e0
[    0.083776] [c00000010d10ba40] [c0000000002e03d0] ___slab_alloc+0x620/0x700
[    0.083822] [c00000010d10bb70] [c0000000002e04e4] __slab_alloc+0x34/0x60
[    0.083868] [c00000010d10bba0] [c0000000002e101c] kmem_cache_alloc_node_trace+0xdc/0x310
[    0.083947] [c00000010d10bc00] [c000000000eb8120] mem_cgroup_init+0x158/0x1c8
[    0.083994] [c00000010d10bc40] [c00000000000dde8] do_one_initcall+0x68/0x1d0
[    0.084041] [c00000010d10bd00] [c000000000e84184] kernel_init_freeable+0x278/0x360
[    0.084094] [c00000010d10bdc0] [c00000000000e714] kernel_init+0x24/0x170
[    0.084143] [c00000010d10be30] [c00000000000c0e8] ret_from_kernel_thread+0x5c/0x74
[    0.084195] Instruction dump:
[    0.084220] eb81ffe0 eba1ffe8 ebc1fff0 ebe1fff8 4e800020 3d230001 e9499a42 3d220004
[    0.084300] 3929acd8 794a1f24 7d295214 eac90100 <e9360000> 2fa90000 419eff74 3b200000
[    0.084382] ---[ end trace 342f5208b00d01b6 ]---

This is a chicken and egg issue where the kernel try to get free
memory when allocating per node data in mem_cgroup_init(), but in that
path mem_cgroup_soft_limit_reclaim() is called which assumes that
these data are allocated.

As mem_cgroup_soft_limit_reclaim() is best effort, it should return
when these data are not yet allocated.

This patch also fixes potential null pointer access in
mem_cgroup_remove_from_trees() and mem_cgroup_update_tree().

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
---
 mm/memcontrol.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 45867e439d31..a9f10fde44a6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -465,6 +465,8 @@ static void mem_cgroup_update_tree(struct mem_cgroup *memcg, struct page *page)
 	struct mem_cgroup_tree_per_node *mctz;
 
 	mctz = soft_limit_tree_from_page(page);
+	if (!mctz)
+		return;
 	/*
 	 * Necessary to update all ancestors when hierarchy is used.
 	 * because their event counter is not touched.
@@ -502,7 +504,8 @@ static void mem_cgroup_remove_from_trees(struct mem_cgroup *memcg)
 	for_each_node(nid) {
 		mz = mem_cgroup_nodeinfo(memcg, nid);
 		mctz = soft_limit_tree_node(nid);
-		mem_cgroup_remove_exceeded(mz, mctz);
+		if (mctz)
+			mem_cgroup_remove_exceeded(mz, mctz);
 	}
 }
 
@@ -2557,7 +2560,7 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
 	 * is empty. Do it lockless to prevent lock bouncing. Races
 	 * are acceptable as soft limit is best effort anyway.
 	 */
-	if (RB_EMPTY_ROOT(&mctz->rb_root))
+	if (!mctz || RB_EMPTY_ROOT(&mctz->rb_root))
 		return 0;
 
 	/*
-- 
2.7.4