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[Xen-devel] bad page flags booting 32bit dom0 on 64bit hypervisor using dom0_mem (kernel >=4.2)

I recently tried to boot 32bit dom0 on 64bit Xen host which I configured to run
with a limited, fix amount of memory for dom0. It seems that somewhere between
kernel versions 3.19 and 4.2 (sorry that is still a wide range) the Linux kernel
would report bad page flags for a range of pages (which seem to be around the
end of the guest pfn range). For a 4.2 kernel that was easily missed as the boot
finished ok and dom0 was accessible. However starting with 4.4 (tested 4.5 and a
4.6-rc) the serial console output freezes after some of those bad page flag
messages and then (unfortunately without any further helpful output) the host
reboots (I assume there is a panic that triggers a reset).

I suspect the problem is more a kernel side one. It is just possible to
influence things by variation of dom0_mem=#,max:#. 512M seems ok, 1024M, 2048M,
and 3072M cause bad page flags starting around kernel 4.2 and reboots around
4.4. Then 4096M and not clamping dom0 memory seem to be ok again (though not
limiting dom0 memory seems to cause trouble on 32bit dom0 later when a domU
tries to balloon memory, but I think that is a different problem).

I have not seen this on a 64bit dom0. Below is an example of those bad page
errors. Somehow it looks to be a page marked as reserved. Initially I wondered
whether this could be a problem of not clearing page flags when moving mappings
to match the e820. But I never looked into i386 memory setup in that detail. So
I am posting this, hoping that someone may have an idea from the detail about
where to look next. PAE is enabled there. Usually its bpf init that gets hit but
that likely is just because that is doing the first vmallocs.


[    4.748815] BUG: Bad page state in process swapper/0  pfn:3fc1e
[    4.748861] page:f675a4b0 count:0 mapcount:0 mapping:  (null) index:0x0
[    4.748908] flags: 0x3000400(reserved)
[    4.748984] page dumped because: PAGE_FLAGS_CHECK_AT_PREP flag set
[    4.749030] bad because of flags:
[    4.749069] flags: 0x400(reserved)
[    4.749143] Modules linked in:
[    4.749201] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G    B
4.2.0-10-generic #12-Ubuntu
[    4.749303] Hardware name: Intel Corporation ...
[    4.749379]  00000000 00000000 f0cffcfc c1730710 f675a4b0 f0cffd20 c115be27
[    4.749584]  f0d503ec 0003fc1e 007fffff c1946eb8 c194a70e 00000001 f0cffd8c
[    4.749790]  00000002 00000141 00009069 c1ac5384 00000000 f11d7ce4 f11d7ce4
[    4.749993] Call Trace:
[    4.750034]  [<c1730710>] dump_stack+0x41/0x52
[    4.750078]  [<c115be27>] bad_page+0xb7/0x110
[    4.750121]  [<c115f4c3>] get_page_from_freelist+0x2d3/0x610
[    4.750168]  [<c115fa26>] __alloc_pages_nodemask+0x146/0x8f0
[    4.750215]  [<c121f1c2>] ? find_entry.isra.13+0x52/0x90
[    4.750260]  [<c11a4825>] ? kmem_cache_alloc_trace+0x175/0x1e0
[    4.750308]  [<c10ab8b6>] ? 
[    4.750373]  [<c11a468d>] ? __kmalloc+0x21d/0x240
[    4.750417]  [<c119097e>] __vmalloc_node_range+0x10e/0x210
[    4.750464]  [<c113efb7>] ? bpf_prog_alloc+0x37/0xa0
[    4.750509]  [<c1190ae6>] __vmalloc_node+0x66/0x70
[    4.750553]  [<c113efb7>] ? bpf_prog_alloc+0x37/0xa0
[    4.750598]  [<c1190b24>] __vmalloc+0x34/0x40
[    4.750642]  [<c113efb7>] ? bpf_prog_alloc+0x37/0xa0
[    4.750687]  [<c113efb7>] bpf_prog_alloc+0x37/0xa0
[    4.750732]  [<c165319c>] bpf_prog_create+0x2c/0x90
[    4.750776]  [<c1b37c34>] ? bsp_pm_check_init+0x11/0x11
[    4.750821]  [<c1b38bf3>] ptp_classifier_init+0x20/0x28
[    4.750866]  [<c1b30042>] ?
[    4.750933]  [<c1b37cb0>] sock_init+0x7c/0x83
[    4.750977]  [<c100211a>] do_one_initcall+0xaa/0x200
[    4.751021]  [<c1b37c34>] ? bsp_pm_check_init+0x11/0x11
[    4.751067]  [<c1080cfd>] ? parse_args+0x2ad/0x540
[    4.751112]  [<c1ae8cd2>] kernel_init_freeable+0x13a/0x1bc
[    4.751158]  [<c1728450>] kernel_init+0x10/0xe0
[    4.751203]  [<c108cba1>] ? schedule_tail+0x11/0x50
[    4.751251]  [<c1735f41>] ret_from_kernel_thread+0x21/0x30
[    4.751297]  [<c1728440>] ? rest_init+0x70/0x70

For reference some memory info from an Intel box with 8G physical memory, booted
with 1024M dom0 memory on a 4.2 kernel. The range of bad page pfn from syslog
was 3fc1e to 3fc3b. The reported pages (f675a4b0 to f675a938) all with the same
line about no mappings or references.

(XEN) Xen-e820 RAM map:
(XEN) 0000000000000000 - 000000000009a400 (usable)
(XEN) 000000000009a400 - 00000000000a0000 (reserved)
(XEN) 00000000000e0000 - 0000000000100000 (reserved)
(XEN) 0000000000100000 - 0000000030a48000 (usable)
(XEN) 0000000030a48000 - 0000000030a49000 (reserved)
(XEN) 0000000030a49000 - 00000000a27f4000 (usable)
(XEN) 00000000a27f4000 - 00000000a2ab4000 (reserved)
(XEN) 00000000a2ab4000 - 00000000a2fb4000 (ACPI NVS)
(XEN) 00000000a2fb4000 - 00000000a2feb000 (ACPI data)
(XEN) 00000000a2feb000 - 00000000a3000000 (usable)
(XEN) 00000000a3000000 - 00000000afa00000 (reserved)
(XEN) 00000000e0000000 - 00000000f0000000 (reserved)
(XEN) 00000000fec00000 - 00000000fec01000 (reserved)
(XEN) 00000000fed00000 - 00000000fed04000 (reserved)
(XEN) 00000000fed10000 - 00000000fed1a000 (reserved)
(XEN) 00000000fed1c000 - 00000000fed20000 (reserved)
(XEN) 00000000fed84000 - 00000000fed85000 (reserved)
(XEN) 00000000fee00000 - 00000000fee01000 (reserved)
(XEN) 00000000ffc00000 - 0000000100000000 (reserved)
(XEN) 0000000100000000 - 000000024e600000 (usable)
(XEN) Xen kernel: 64-bit, lsb, compat32
(XEN) Dom0 kernel: 32-bit, PAE, lsb, paddr 0x1000000 -> 0x1ecc000
(XEN) Dom0 alloc.: 0000000240000000->0000000244000000 (238639 pages to be all
(XEN) Init. ramdisk: 000000024ca2f000->000000024e5ffbd4
(XEN) Loaded kernel: 00000000c1000000->00000000c1ecc000
(XEN) Init. ramdisk: 0000000000000000->0000000000000000
(XEN) Phys-Mach map: 00000000c1ecc000->00000000c1fcc000
(XEN) Start info: 00000000c1fcc000->00000000c1fcc4b4
(XEN) Page tables: 00000000c1fcd000->00000000c1fe4000
(XEN) Boot stack: 00000000c1fe4000->00000000c1fe5000
(XEN) TOTAL: 00000000c0000000->00000000c2400000
(XEN) ENTRY ADDRESS: 00000000c1ae8254

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