Re: [Xen-devel] [ 3009.778974] mcelog:16842 map pfn expected mapping type write-back for [mem 0x0009f000-0x000a0fff], got uncached-minus

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Nov 16, 2012 at 01:40:56PM +0100, Sander Eikelenboom wrote:
> Hi Konrad,
> 
> Sometime ago i reported this one at boot up:
> 
> [ 3009.778974] mcelog:16842 map pfn expected mapping type write-back for [mem 
> 0x0009f000-0x000a0fff], got uncached-minus
> [ 3009.788570] ------------[ cut here ]------------
> [ 3009.798175] WARNING: at arch/x86/mm/pat.c:774 untrack_pfn+0xa1/0xb0()
> [ 3009.807966] Hardware name: MS-7640
> [ 3009.817677] Modules linked in:
> [ 3009.827524] Pid: 16842, comm: mcelog Tainted: G        W    
> 3.7.0-rc5-20121116-reverted-persistent-warn-patwarn #1
> [ 3009.837415] Call Trace:
> [ 3009.847110]  [<ffffffff810674fa>] warn_slowpath_common+0x7a/0xb0
> [ 3009.856857]  [<ffffffff81067545>] warn_slowpath_null+0x15/0x20
> [ 3009.866562]  [<ffffffff81042041>] untrack_pfn+0xa1/0xb0
> [ 3009.876201]  [<ffffffff8111a59b>] unmap_single_vma+0x86b/0x8e0
> [ 3009.885895]  [<ffffffff81100f16>] ? release_pages+0x196/0x1f0
> [ 3009.895488]  [<ffffffff8111a65c>] unmap_vmas+0x4c/0xa0
> [ 3009.905134]  [<ffffffff8111c8fa>] exit_mmap+0x9a/0x180
> [ 3009.914706]  [<ffffffff81064e72>] mmput+0x52/0xd0
> [ 3009.924252]  [<ffffffff810652b7>] dup_mm+0x3c7/0x510
> [ 3009.933839]  [<ffffffff81065fd5>] copy_process+0xac5/0x14a0
> [ 3009.943430]  [<ffffffff81066af3>] do_fork+0x53/0x360
> [ 3009.952843]  [<ffffffff810b25c7>] ? lock_release+0x117/0x250
> [ 3009.962283]  [<ffffffff817d26c0>] ? _raw_spin_unlock+0x30/0x60
> [ 3009.971532]  [<ffffffff817d3495>] ? sysret_check+0x22/0x5d
> [ 3009.980820]  [<ffffffff81017523>] sys_clone+0x23/0x30
> [ 3009.990046]  [<ffffffff817d37f3>] stub_clone+0x13/0x20
> [ 3009.999335]  [<ffffffff817d3469>] ? system_call_fastpath+0x16/0x1b
> [ 3010.008667] ---[ end trace 2d9694c2c0a24da8 ]---
> 
> 
> It seems to be due to the "mcelog" userspace tool provided with Debian 
> Squeeze (mcelog 1.0~pre3-3  x86-64 Machine Check Exceptions collector and 
> decoder).
> I can trigger this warning easily by restarting the mcelog tool with 
> /etc/init.d/mcelog restart
> 
> Should that one also function with the xen mcelog driver, or is a newer 
> version required ?

The reason we get is b/c I had to disable the PAT functionality in the Linux 
kernel for Xen.
This is b/c it only worked one way - meaning you could convert a page from
WriteBack to WriteCombine or WriteBack to Uncached. But you could not
do WriteCombine back to WriteBack - due to one of the functions that
changes the bits was using an "unfiltered" way to identify the bits on the
page.

Anyhow, we had a disaster b/c some of these pages that used to WriteBack (WB)
got converted to WriteCombine (WC) and then were returned back as such
to the page pool. And if they were re-used by filesystem invariably we got
corruptions.

So until the PAT table lookup thing that Peter H. Anvin suggested
gets implemented this splat gotta show up :-(

Does mcelog still work even with this warning?
> 
> --
> Sander

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