On Fri 14-12-18 23:17:26, Rik van Riel wrote:
> Changeset 9b6f7e163cd0 ("mm: rework memcg kernel stack accounting")
> will result in fork failing if allocating a kernel stack for a task
> in dup_task_struct exceeds the kernel memory allowance for that cgroup.
>
> Unfortunately, it also results in a crash.
>
> This is due to the code jumping to free_stack and calling free_thread_stack
> when the memcg kernel stack charge fails, but without tsk->stack pointing
> at the freshly allocated stack.
>
> This in turn results in the vfree_atomic in free_thread_stack oopsing
> with a backtrace like this:
>
> #5 [ffffc900244efc88] die at ffffffff8101f0ab
> #6 [ffffc900244efcb8] do_general_protection at ffffffff8101cb86
> #7 [ffffc900244efce0] general_protection at ffffffff818ff082
> [exception RIP: llist_add_batch+7]
> RIP: ffffffff8150d487 RSP: ffffc900244efd98 RFLAGS: 00010282
> RAX: 0000000000000000 RBX: ffff88085ef55980 RCX: 0000000000000000
> RDX: ffff88085ef55980 RSI: 343834343531203a RDI: 343834343531203a
> RBP: ffffc900244efd98 R8: 0000000000000001 R9: ffff8808578c3600
> R10: 0000000000000000 R11: 0000000000000001 R12: ffff88029f6c21c0
> R13: 0000000000000286 R14: ffff880147759b00 R15: 0000000000000000
> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
> #8 [ffffc900244efda0] vfree_atomic at ffffffff811df2c7
> #9 [ffffc900244efdb8] copy_process at ffffffff81086e37
> #10 [ffffc900244efe98] _do_fork at ffffffff810884e0
> #11 [ffffc900244eff10] sys_vfork at ffffffff810887ff
> #12 [ffffc900244eff20] do_syscall_64 at ffffffff81002a43
> RIP: 000000000049b948 RSP: 00007ffcdb307830 RFLAGS: 00000246
> RAX: ffffffffffffffda RBX: 0000000000896030 RCX: 000000000049b948
> RDX: 0000000000000000 RSI: 00007ffcdb307790 RDI: 00000000005d7421
> RBP: 000000000067370f R8: 00007ffcdb3077b0 R9: 000000000001ed00
> R10: 0000000000000008 R11: 0000000000000246 R12: 0000000000000040
> R13: 000000000000000f R14: 0000000000000000 R15: 000000000088d018
> ORIG_RAX: 000000000000003a CS: 0033 SS: 002b
>
> The simplest fix is to assign tsk->stack right where it is allocated.
>
> Fixes: 9b6f7e163cd0 ("mm: rework memcg kernel stack accounting")
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
Ouch, I completely missed this during the review. The code is quite
subtle. I was about to suggest that we simply do
tsk->stack = alloc_thread_stack_node(tsk, node);
but arch_dup_task_struct overwrites it and that is the reason we have
this hairy handling and assign tsk->stack 2 times.
The patch doesn't improve the overall readbility but it is a fix so
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> kernel/fork.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 07cddff89c7b..e2a5156bc9c3 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -240,8 +240,10 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
> * free_thread_stack() can be called in interrupt context,
> * so cache the vm_struct.
> */
> - if (stack)
> + if (stack) {
> tsk->stack_vm_area = find_vm_area(stack);
> + tsk->stack = stack;
> + }
> return stack;
> #else
> struct page *page = alloc_pages_node(node, THREADINFO_GFP,
> @@ -288,7 +290,10 @@ static struct kmem_cache *thread_stack_cache;
> static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
> int node)
> {
> - return kmem_cache_alloc_node(thread_stack_cache, THREADINFO_GFP, node);
> + unsigned long *stack;
> + stack = kmem_cache_alloc_node(thread_stack_cache, THREADINFO_GFP, node);
> + tsk->stack = stack;
> + return stack;
> }
>
> static void free_thread_stack(struct task_struct *tsk)
>
>
> --
> All rights reversed.
--
Michal Hocko
SUSE Labs
