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    SubjectRe: [PATCH 1/2] ARC: show_regs: avoid page allocator
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    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@synopsys.com>
    > ---
    > 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
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