Re: [PATCH 00/16] The new slab memory controller

[Bharata B Rao]

Hi,

I see the below crash during early boot when I try this patchset on
PowerPC host. I am on new_slab.rfc.v5.3 branch.

BUG: Unable to handle kernel data access at 0x81030236d1814578
Faulting instruction address: 0xc0000000002cc314
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA PowerNV
Modules linked in: ip_tables x_tables autofs4 sr_mod cdrom usbhid bnx2x crct10dif_vpmsum crct10dif_common mdio libcrc32c crc32c_vpmsum
CPU: 31 PID: 1752 Comm: keyboard-setup. Not tainted 5.3.0-g9bd85fd72a0c #155
NIP:  c0000000002cc314 LR: c0000000002cc2e8 CTR: 0000000000000000
REGS: c000001e40f378b0 TRAP: 0380   Not tainted  (5.3.0-g9bd85fd72a0c)
MSR:  900000010280b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE,TM[E]>  CR: 44022224  XER: 00000000
CFAR: c0000000002c6ad4 IRQMASK: 1 
GPR00: c0000000000b8a40 c000001e40f37b40 c000000000ed9600 0000000000000000 
GPR04: 0000000000000023 0000000000000010 c000001e40f37b24 c000001e3cba3400 
GPR08: 0000000000000020 81030218815f4578 0000001e50220000 0000000000000030 
GPR12: 0000000000002200 c000001fff774d80 0000000000000000 00000001072600d8 
GPR16: 0000000000000000 c0000000000bbaac 0000000000000000 0000000000000000 
GPR20: c000001e40f37c48 0000000000000001 0000000000000000 c000001e3cba3400 
GPR24: c000001e40f37dd8 0000000000000000 c000000000fa0d58 0000000000000000 
GPR28: c000001e3a080080 c000001e32da0100 0000000000000118 0000000000000010 
NIP [c0000000002cc314] __mod_memcg_state+0x58/0xd0
LR [c0000000002cc2e8] __mod_memcg_state+0x2c/0xd0
Call Trace:
[c000001e40f37b90] [c0000000000b8a40] account_kernel_stack+0xa4/0xe4
[c000001e40f37bd0] [c0000000000ba4a4] copy_process+0x2b4/0x16f0
[c000001e40f37cf0] [c0000000000bbaac] _do_fork+0x9c/0x3e4
[c000001e40f37db0] [c0000000000bc030] sys_clone+0x74/0xa8
[c000001e40f37e20] [c00000000000bb34] ppc_clone+0x8/0xc
Instruction dump:
4bffa7e9 2fa30000 409e007c 395efffb 3d000020 2b8a0001 409d0008 39000020 
e93d0718 e94d0028 7bde1f24 7d29f214 <7ca9502a> 7fff2a14 7fe9fe76 7d27fa78 

Looks like page->mem_cgroup_vec is allocated but not yet initialized
with memcg pointers when we try to access them.

I did get past the crash by initializing the pointers like this
in account_kernel_stack(), but I am pretty sure that this is not the
place to do this:

diff --git a/kernel/fork.c b/kernel/fork.c
index 541fd805fb88..be21419feae2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -380,13 +380,26 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
                 * All stack pages are in the same zone and belong to the
                 * same memcg.
                 */
-               struct page *first_page = virt_to_page(stack);
+               struct page *first_page = virt_to_head_page((stack));
+               unsigned long off;
+               struct mem_cgroup_ptr *memcg_ptr;
+               struct mem_cgroup *memcg;
 
                mod_zone_page_state(page_zone(first_page), NR_KERNEL_STACK_KB,
                                    THREAD_SIZE / 1024 * account);
 
-               mod_memcg_page_state(first_page, MEMCG_KERNEL_STACK_KB,
+               if (!first_page->mem_cgroup_vec)
+                       return;
+               off = obj_to_index(task_struct_cachep, first_page, stack);
+               memcg_ptr = first_page->mem_cgroup_vec[off];
+               if (!memcg_ptr)
+                       return;
+               rcu_read_lock();
+               memcg = memcg_ptr->memcg;
+               if (memcg)
+                       __mod_memcg_state(memcg, MEMCG_KERNEL_STACK_KB,
                                     account * (THREAD_SIZE / 1024));
+               rcu_read_unlock();
        }
 }

Regards,
Bharata.