linux-kernel - [PATCH 0/5] Persist printk buffer across reboots.

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Date:	Tue, 13 Mar 2012 01:36:36 -0400
From:	Avery Pennarun <apenwarr@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Fabio M. Di Nitto" <fdinitto@...hat.com>,
	Avery Pennarun <apenwarr@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Olaf Hering <olaf@...fle.de>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Tejun Heo <tj@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Yinghai LU <yinghai@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [PATCH 0/5] Persist printk buffer across reboots.

The last patch in this series implements a new CONFIG_PRINTK_PERSIST option
that, when enabled, puts the printk buffer in a well-defined memory location
so that we can keep appending to it after a reboot.  The upshot is that,
even after a kernel panic or non-panic hard lockup, on the next boot
userspace will be able to grab the kernel messages leading up to it.  It
could then upload the messages to a server (for example) to keep crash
statistics.

The preceding patches in the series are mostly just things I fixed up while
working on that patch.

Some notes:

- I'm not totally sure of the locking or portability issues when calling
  memblock or bootmem.  This all happens really early, and I *think*
  interrupts are still disabled at that time, so it's probably okay.

- Tested this version on x86 (kvm) and it works with soft reboot (ie. reboot
  -f).  Since some BIOSes wipe the memory during boot, you might not have
  any luck.  It should be great on many embedded systems, though, including
  the MIPS system I've tested a variant of this patch on.  (Our MIPS build
  is based on a slightly older kernel so it's not 100% the same, but I think
  this should behave identically.)
  
- The way we choose a well-defined memory location is slightly suspicious
  (we just count down from the top of the address space) but I've tested it
  pretty carefully, and it seems to be okay.

- In printk.c with CONFIG_PRINTK_PERSIST set, we're #defining words like
  log_end.  It might be cleaner to replace all instances of log_end with
  LOG_END to make this more clear.  This is also the reason the struct
  logbits members start with _: because otherwise they conflict with the
  macro.  Suggestions welcome.

Patch generated against v3.3-rc7.

Avery Pennarun (5):
  mm: bootmem: BUG() if you try to allocate bootmem too late.
  mm: bootmem: it's okay to reserve_bootmem an invalid address.
  mm: nobootmem: implement reserve_bootmem() in terms of memblock.
  printk: use alloc_bootmem() instead of memblock_alloc().
  printk: CONFIG_PRINTK_PERSIST: persist printk buffer across reboots.

 init/Kconfig    |   12 ++++++
 kernel/printk.c |  117 +++++++++++++++++++++++++++++++++++++++++++++---------
 mm/bootmem.c    |   10 ++++-
 mm/nobootmem.c  |   23 +++++++++++
 4 files changed, 140 insertions(+), 22 deletions(-)

-- 
1.7.7.3

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