Date:	Thu, 29 Jul 2010 13:22:16 -0600
From:	dann frazier <dannf@...ian.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>, Mel Gorman <mel@....ul.ie>,
	Minchan Kim <minchan.kim@...il.com>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: ia64 hang/mca running gdb 'make check'

On Wed, Jul 28, 2010 at 08:50:18PM -0700, Hugh Dickins wrote:
> On Tue, 27 Jul 2010, dann frazier wrote:
> > On Tue, Jul 27, 2010 at 06:03:30PM +0900, KAMEZAWA Hiroyuki wrote:
> > > On Tue, 27 Jul 2010 01:19:15 -0600
> > > dann frazier <dannf@...ian.org> wrote:
> > > > On Tue, Jul 20, 2010 at 09:19:50PM -0700, Hugh Dickins wrote:
> > > > > On Tue, 20 Jul 2010, dann frazier wrote:
> > > > > > On Wed, Jul 21, 2010 at 10:51:36AM +0900, KAMEZAWA Hiroyuki wrote:
> > > > > > > On Tue, 20 Jul 2010 11:35:12 -0600
> > > > > > > dann frazier <dannf@...ian.org> wrote:
> > > > > > > 
> > > > > > > > Debian's ia64 autobuilders have been experiencing system crashes while
> > > > > > > > trying to run the gdb test suite:
> > > > > > > >   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=588574
> > > > > > > > 
> > > > > > > > I was able to reproduce this w/ the latest git tree, and bisected it
> > > > > > > > down to this commit, introduced in 2.6.32:
> > > > > > > > 
> > > > > > > >   commit 62eede62dafb4a6633eae7ffbeb34c60dba5e7b1
> > > > > > > >   Author: Hugh Dickins <hugh.dickins@...cali.co.uk>
> > > > > > > >   Date:   Mon Sep 21 17:03:34 2009 -0700
> > > > > > > > 
> > > > > > > >     mm: ZERO_PAGE without PTE_SPECIAL
> > > > > > > > 
> > > > > > > >     Reinstate anonymous use of ZERO_PAGE to all architectures, not just to
> > > > > > > >     those which __HAVE_ARCH_PTE_SPECIAL: as suggested by Nick Piggin.
> > > > > > > > 
> > > > > > > >     Contrary to how I'd imagined it, there's nothing ugly about this, just a
> > > > > > > >     zero_pfn test built into one or another block of vm_normal_page().
> > > > > > > > 
> > > > > > > >     But the MIPS ZERO_PAGE-of-many-colours case demands is_zero_pfn() and
> > > > > > > >     my_zero_pfn() inlines.  Reinstate its mremap move_pte() shuffling of
> > > > > > > >     ZERO_PAGEs we did from 2.6.17 to 2.6.19?  Not unless someone shouts for
> > > > > > > >     that: it would have to take vm_flags to weed out some cases.
> > > > > > > > 
> > > > > > > > fyi, I found this to not be reproducible on SLES11 SP1 (which is
> > > > > > > > 2.6.32-based). I compared the .configs and found that the relevant
> > > > > > > > difference is the PAGE_SIZE. It does not fail w/ 64KB pages, but
> > > > > > > > reliably fails w/ 16KB pages.
> > > > > > > > 
> > > > > > > 
> > > > > > > Sorry, I have no idea...
> > > > > > > Hmm, what is the address of empty_zero_page[] on your debian(16kb-page) ?
> > > > > > 
> > > > > > 
> > > > > > dannf@...bs:~$ grep empty_zero_page /boot/System.map-2.6.32-5-mckinley 
> > > > > > a0000001008784c0 d __ksymtab_empty_zero_page
> > > > > > a000000100882688 d __kcrctab_empty_zero_page
> > > > > > a000000100884ca4 r __kstrtab_empty_zero_page
> > > > > > a000000100974000 D empty_zero_page
> > > > > 
> > > > > Thanks a lot for reporting this, but I too have no idea yet.
> > > > > 
> > > > > It is likely that the bug is not to be found in that 62eede62, but
> > > > > rather in one of the preceding patches to mm/memory.c which 62eede62
> > > > > was extending to ia64 and other architectures without PTE_SPECIAL.
> > > > > 
> > > > > I wonder, from looking at that gdb testsuite log, is it plausible
> > > > > that all these hangs/crashes occurred when writing out a coredump?
> > > > > Is that something you could check for us? or rule out the possibility.
> > > > 
> > > > Yep, seems so. I've reduced it down to this test case:
> > > > 
> > > > dannf@...600:~> cat > foo.c
> > > > int leaf(void) {
> > > >   return 0;
> > > > }
> > > > 
> > > > int main(void) {
> > > >   leaf();
> > > > }
> > > > dannf@...600:~> gcc -g foo.c -o foo
> > > > dannf@...600:~> gdb ./foo 
> > > > GNU gdb (GDB) SUSE (7.0-0.4.16)
> > > > Copyright (C) 2009 Free Software Foundation, Inc.
> > > > License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> > > > This is free software: you are free to change and redistribute it.
> > > > There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
> > > > and "show warranty" for details.
> > > > This GDB was configured as "ia64-suse-linux".
> > > > For bug reporting instructions, please see:
> > > > <http://www.gnu.org/software/gdb/bugs/>...
> > > > Reading symbols from /home/dannf/foo...done.
> > > > (gdb) break leaf
> > > > Breakpoint 1 at 0x40000000000005c1: file foo.c, line 2.
> > > > (gdb) run
> > > > Starting program: /home/dannf/foo 
> > > > Missing separate debuginfo for /lib/ld-linux-ia64.so.2
> > > > Try: zypper install -C "debuginfo(build-id)=d5bfb8b5940e174d54b978ca515dc0df76c7618c"
> > > > Missing separate debuginfo for /lib/libc.so.6.1
> > > > Try: zypper install -C "debuginfo(build-id)=ca78657bd9173653d95f8504a313d2b6db8cb1d6"
> > > > 
> > > > Breakpoint 1, leaf () at foo.c:2
> > > > 2	     return 0;
> > > > (gdb) gcore /tmp/save
> > > > 
> > > > [bang]
> > > > 
> > > 
> > > Does this happen on 2.6.34 or 2.6.35-rc kernel ?
> > 
> > I've been testing w/ a 2.6.35-rc4+, though it was originally reported
> > on a 2.6.32.
> 
> Thanks a lot for narrowing down to that simple testcase, and
> thanks a lot for checking it's just as bad on recent kernels.
> 
> I'm sorry to say that I'm still just as baffled.
> 
> Let's note that gdb's gcore is building up its own version of a
> coredump, not going through the get_dump_page() code I was wondering
> about.  If I read gcore correctly (possibly not!), it will be reading
> selected areas from /proc/<pid>/mem i.e. using access_process_vm().

This appears to be correct. I was able to collect the following
stacktrace using INIT:

[ 2535.074197] Backtrace of pid 4605 (gdb)
[ 2535.074197] 
[ 2535.074197] Call Trace:
[ 2535.074197]  [<a00000010000bb00>] ia64_native_leave_kernel+0x0/0x270
[ 2535.074197]                                 sp=e000004081c77c40 bsp=e000004081c71018
[ 2535.074197]  [<a000000100334720>] __copy_user+0x160/0x960
[ 2535.074197]                                 sp=e000004081c77e10 bsp=e000004081c71018
[ 2535.074197]  [<a000000100176b00>] access_process_vm+0x2c0/0x380
[ 2535.074197]                                 sp=e000004081c77e10 bsp=e000004081c70f60

> But why the (16kB but not 64kB!) zero page should make that freeze
> or reboot, I have no idea.
> 
> What would I be doing if I had an Itanium?  I think I'd be trying to
> narrow down exactly where it goes bad (tedious when the penalty is
> a freeze or reboot).
> 
> As it is, I'm hoping that someone with an ia64 can investigate...
> 
> Hugh
> 

-- 
dann frazier

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