linux-kernel - Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support

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Date:   Tue, 30 Aug 2016 17:26:12 -0500
From:   Josh Cartwright <joshc@...com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v6 1/3] fork: Add generic vmalloced stack support

On Tue, Aug 30, 2016 at 03:01:51PM -0700, Andy Lutomirski wrote:
> On Wed, Aug 24, 2016 at 9:51 AM, Josh Cartwright <joshc@...com> wrote:
[..]
> >> diff --git a/kernel/fork.c b/kernel/fork.c
> >> index 52e725d4a866..05f7ef796fb4 100644
> >> --- a/kernel/fork.c
> >> +++ b/kernel/fork.c
> >> @@ -158,19 +158,39 @@ void __weak arch_release_thread_stack(unsigned long *stack)
> >>   * Allocate pages if THREAD_SIZE is >= PAGE_SIZE, otherwise use a
> >>   * kmemcache based allocator.
> >>   */
> >> -# if THREAD_SIZE >= PAGE_SIZE
> >> -static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
> >> -                                               int node)
> >> +# if THREAD_SIZE >= PAGE_SIZE || defined(CONFIG_VMAP_STACK)
> >> +static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
> >>  {
> >> +#ifdef CONFIG_VMAP_STACK
> >> +     void *stack = __vmalloc_node_range(THREAD_SIZE, THREAD_SIZE,
> >> +                                        VMALLOC_START, VMALLOC_END,
> >> +                                        THREADINFO_GFP | __GFP_HIGHMEM,
> >> +                                        PAGE_KERNEL,
> >> +                                        0, node,
> >> +                                        __builtin_return_address(0));
> >> +
> >> +     /*
> >> +      * We can't call find_vm_area() in interrupt context, and
> >> +      * free_thread_stack can be called in interrupt context, so cache
> >> +      * the vm_struct.
> >> +      */
> >> +     if (stack)
> >> +             tsk->stack_vm_area = find_vm_area(stack);
> >
> > This is annoying, we end up having to walk the vm_area tree twice (once
> > for the allocation, then here to get a handle on area).
> >
> > Perhaps it's time the vmalloc code learned an allocation API that
> > returned the vm_area handle as well.
> 
> Agreed.  I may do this once everything else lands.

There are at least a few other places that could benefit from this,
doing a quick scan of find_vm_area() callers: vmalloc_{32_,}_user() and
kasan_module_alloc().

  Josh

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