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Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c

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On 06/21, Linus Torvalds wrote:
>
> Hugh, Michal - I also merged Helge's drop-up cleanup, is there
> anything I've missed? I think Oleg had something, but I can't recall
> right now, and I might just have missed it.

Well, I meant, perhaps we need a bit more changes to ensure that a new
GROWSDOWN vma can't come without a gap below. But this is really minor,
we can do this later even if I am right.

However, there is another regression reported by Cyrill. Fixed by the
patch below.

And yes, I think this check should either go away, or we need to make
it more clever.

In short, the vma created by mmap(MAP_GROWSDOWN) does not grow down
automatically, because of this check.

This worked before, because with the stack guard page at ->vm_start
__do_page_fault() hits this expand-stack path only if the stack grows
by more than PAGE_SIZE, now it is called every time. I'll send the
patch tomorrow if nobody else does this before.

Oleg.
---

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 8ad91a0..edc5d68 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1416,7 +1416,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
 		 * and pusha to work. ("enter $65535, $31" pushes
 		 * 32 pointers and then decrements %sp by 65535.)
 		 */
-		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
+if (0)		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
 			bad_area(regs, error_code, address);
 			return;
 		}

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Re: [lkp-robot] [mm]  1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-21 04:50 +0200
  Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-21 20:30 +0200
    Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-21 21:40 +0200
      Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-21 22:30 +0200
        Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-21 22:40 +0200
          Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-21 23:00 +0200
            Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Linus Torvalds <torvalds@linux-foundation.org> - 2017-06-22 00:20 +0200
              Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-22 03:10 +0200
                Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-22 17:20 +0200
                Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-22 20:10 +0200
                Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-22 23:00 +0200
    Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Oleg Nesterov <oleg@redhat.com> - 2017-06-21 21:40 +0200
      Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-22 06:30 +0200
    Re: [lkp-robot] [mm] 1be7107fbe: kernel_BUG_at_mm/mmap.c Hugh Dickins <hughd@google.com> - 2017-06-21 21:50 +0200

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