Re: [PATCH 0/2 x86] fix some page faults in nmi if kmemcheck isenabled

[Li Zhong]

On Mon, 2012-02-20 at 12:00 +0100, Peter Zijlstra wrote:
> On Mon, 2012-02-20 at 14:01 +0800, Li Zhong wrote:
> > If CONFIG_KMEMCHECK is enabled, there might be page faults in nmi if the
> > pages are marked as not present by kmemcheck, like following:
> > 
> > [    4.535803] WARNING: at arch/x86/mm/kmemcheck/kmemcheck.c:634 kmemcheck_fault+0xb9/0xd0()
> > [    4.633429] Hardware name: System x3650 M3 -[7945AC1]-
> > [    4.694710] Modules linked in:
> > [    4.731105] Pid: 1, comm: swapper/0 Not tainted 3.3.0-rc3 #15
> > [    4.799654] Call Trace:
> > [    4.828751]  <NMI>  [<ffffffff81042eca>] warn_slowpath_common+0x7a/0xb0
> > [    4.907713]  [<ffffffff81042f15>] warn_slowpath_null+0x15/0x20
> > [    4.977301]  [<ffffffff8103ce89>] kmemcheck_fault+0xb9/0xd0
> > [    5.043778]  [<ffffffff81551ba6>] do_page_fault+0x406/0x550
> > [    5.110252]  [<ffffffff8154e235>] page_fault+0x25/0x30
> > [    5.171535]  [<ffffffff8154f005>] ? nmi_handle.clone.1+0x75/0xc0
> > [    5.243202]  [<ffffffff8154efcf>] ? nmi_handle.clone.1+0x3f/0xc0
> > [    5.314867]  [<ffffffff8154ef90>] ? __die+0xf0/0xf0
> > [    5.373038]  [<ffffffff8154f15f>] do_nmi+0x10f/0x360
> > [    5.432243]  [<ffffffff8154e5cd>] restart_nmi+0x1a/0x1e
> > [    5.494565]  [<ffffffff8154e210>] ? general_protection+0x30/0x30
> > [    5.566234]  [<ffffffff8154e210>] ? general_protection+0x30/0x30
> > [    5.637898]  [<ffffffff8154e210>] ? general_protection+0x30/0x30
> > [    5.709566]  <<EOE>>  [<ffffffff8126d814>] ? rb_insert_color+0xa4/0x150
> > [    5.788526]  [<ffffffff8119d17b>] sysfs_link_sibling+0x8b/0x110
> > [    5.859155]  [<ffffffff8119dff1>] __sysfs_add_one+0xc1/0x100
> > [    5.926666]  [<ffffffff8119e056>] sysfs_add_one+0x26/0xd0
> > [    5.991065]  [<ffffffff8119cdf4>] sysfs_add_file_mode+0xc4/0x100
> > [    6.062731]  [<ffffffff8119fc41>] internal_create_group+0xc1/0x1a0
> > [    6.136473]  [<ffffffff8119fd4e>] sysfs_create_group+0xe/0x10
> > [    6.205026]  [<ffffffff81351c1a>] dpm_sysfs_add+0x2a/0xd0
> > [    6.269425]  [<ffffffff81349bf5>] device_add+0x5e5/0x730
> > [    6.332783]  [<ffffffff81349d59>] device_register+0x19/0x20
> > [    6.399260]  [<ffffffff8135b6b8>] add_memory_section+0x158/0x1e0
> > [    6.470927]  [<ffffffff81ca757e>] memory_dev_init+0x75/0x108
> > [    6.538439]  [<ffffffff81ca73a9>] driver_init+0x31/0x33
> > [    6.600762]  [<ffffffff81c72c68>] kernel_init+0xcc/0x169
> > [    6.664121]  [<ffffffff81555e64>] kernel_thread_helper+0x4/0x10
> > [    6.734749]  [<ffffffff81c72b9c>] ? start_kernel+0x3ab/0x3ab
> > [    6.802261]  [<ffffffff81555e60>] ? gs_change+0x13/0x13
> > [    6.864585] ---[ end trace a7919e7f17c0a725 ]---
> > 
> > These two patches tries to fix some of the problems by avoiding using the
> > non-present pages.
> 
> 
> Hell no, these are some of the ugliest patches I've seen in a while. Not
> to mention that their changelogs are utter crap since they don't even
> explain why they're doing what they're doing.
> 
Hi Peter, 

I agree that the fix is ugly. I'm willing to change if there are some
better ways. 

The problem here is: 
1. It seems x86 doesn't allow page faults in nmi, and there are checks
in the code, like WARN_ON_ONCE(in_nmi()).
 
2. If CONFIG_KMEMCHECK is enabled, the pages allocated through slab will
be marked as non-present, to capture uninitialized memory access. More
information in Documentation/kmemcheck.txt .

3. From the log, there are some memories accessed in nmi, which are in
pages marked as non-present by kmemcheck, as they are allocated by
something like kmalloc(). 

Thanks,
Zhong

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