Re: [PATCH 1/2] ARC: show_regs: avoid page allocator

[Eugeniy Paltsev]

Hi Vineet,

Just curious: isn't that enough to use GFP_NOWAIT instead
of GFP_KERNEL when we allocate page in show_regs()?

As I can see x86 use print_vma_addr() in their show_signal_msg()
function which allocate page with __get_free_page(GFP_NOWAIT);

On Tue, 2018-12-18 at 10:53 -0800, Vineet Gupta wrote:
> Use on-stack smaller buffers instead of dynamic pages.
> 
> The motivation for this change was to address lockdep splat when
> signal handling code calls show_regs (with preemption disabled) and
> ARC show_regs calls into sleepable page allocator.
> 
> > potentially unexpected fatal signal 11.
> > BUG: sleeping function called from invalid context at ../mm/page_alloc.c:4317
> > in_atomic(): 1, irqs_disabled(): 0, pid: 57, name: segv
> > no locks held by segv/57.
> > Preemption disabled at:
> > [<8182f17e>] get_signal+0x4a6/0x7c4
> > CPU: 0 PID: 57 Comm: segv Not tainted 4.17.0+ #23
> > 
> > Stack Trace:
> >  arc_unwind_core.constprop.1+0xd0/0xf4
> >  __might_sleep+0x1f6/0x234
> >  __get_free_pages+0x174/0xca0
> >  show_regs+0x22/0x330
> >  get_signal+0x4ac/0x7c4     # print_fatal_signals() -> preempt_disable()
> >  do_signal+0x30/0x224
> >  resume_user_mode_begin+0x90/0xd8
> 
> Despite this, lockdep still barfs (see next change), but this patch
> still has merit as in we use smaller/localized buffers now and there's
> less instructoh trace to sift thru when debugging pesky issues.
> 
> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> ---
>  arch/arc/kernel/troubleshoot.c | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arc/kernel/troubleshoot.c b/arch/arc/kernel/troubleshoot.c
> index e8d9fb452346..2885bec71fb8 100644
> --- a/arch/arc/kernel/troubleshoot.c
> +++ b/arch/arc/kernel/troubleshoot.c
> @@ -58,11 +58,12 @@ static void show_callee_regs(struct callee_regs *cregs)
>  	print_reg_file(&(cregs->r13), 13);
>  }
>  
> -static void print_task_path_n_nm(struct task_struct *tsk, char *buf)
> +static void print_task_path_n_nm(struct task_struct *tsk)
>  {
>  	char *path_nm = NULL;
>  	struct mm_struct *mm;
>  	struct file *exe_file;
> +	char buf[256];
>  
>  	mm = get_task_mm(tsk);
>  	if (!mm)
> @@ -80,10 +81,9 @@ static void print_task_path_n_nm(struct task_struct *tsk, char *buf)
>  	pr_info("Path: %s\n", !IS_ERR(path_nm) ? path_nm : "?");
>  }
>  
> -static void show_faulting_vma(unsigned long address, char *buf)
> +static void show_faulting_vma(unsigned long address)
>  {
>  	struct vm_area_struct *vma;
> -	char *nm = buf;
>  	struct mm_struct *active_mm = current->active_mm;
>  
>  	/* can't use print_vma_addr() yet as it doesn't check for
> @@ -96,8 +96,11 @@ static void show_faulting_vma(unsigned long address, char *buf)
>  	 * if the container VMA is not found
>  	 */
>  	if (vma && (vma->vm_start <= address)) {
> +		char buf[256];
> +		char *nm = "?";
> +
>  		if (vma->vm_file) {
> -			nm = file_path(vma->vm_file, buf, PAGE_SIZE - 1);
> +			nm = file_path(vma->vm_file, buf, 256-1);
>  			if (IS_ERR(nm))
>  				nm = "?";
>  		}
> @@ -173,13 +176,8 @@ void show_regs(struct pt_regs *regs)
>  {
>  	struct task_struct *tsk = current;
>  	struct callee_regs *cregs;
> -	char *buf;
> -
> -	buf = (char *)__get_free_page(GFP_KERNEL);
> -	if (!buf)
> -		return;
>  
> -	print_task_path_n_nm(tsk, buf);
> +	print_task_path_n_nm(tsk);
>  	show_regs_print_info(KERN_INFO);
>  
>  	show_ecr_verbose(regs);
> @@ -189,7 +187,7 @@ void show_regs(struct pt_regs *regs)
>  		(void *)regs->blink, (void *)regs->ret);
>  
>  	if (user_mode(regs))
> -		show_faulting_vma(regs->ret, buf); /* faulting code, not data */
> +		show_faulting_vma(regs->ret); /* faulting code, not data */
>  
>  	pr_info("[STAT32]: 0x%08lx", regs->status32);
>  
> @@ -221,8 +219,6 @@ void show_regs(struct pt_regs *regs)
>  	cregs = (struct callee_regs *)current->thread.callee_reg;
>  	if (cregs)
>  		show_callee_regs(cregs);
> -
> -	free_page((unsigned long)buf);
>  }
>  
>  void show_kernel_fault_diag(const char *str, struct pt_regs *regs,
-- 
 Eugeniy Paltsev