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SubjectRe: [PATCH v1 1/3] mm: fix uninitialized memmaps on a partially populated last section
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On 09.12.19 22:15, Daniel Jordan wrote:
> Hi David,
>
> On Mon, Dec 09, 2019 at 06:48:34PM +0100, David Hildenbrand wrote:
>> If max_pfn is not aligned to a section boundary, we can easily run into
>> BUGs. This can e.g., be triggered on x86-64 under QEMU by specifying a
>> memory size that is not a multiple of 128MB (e.g., 4097MB, but also
>> 4160MB). I was told that on real HW, we can easily have this scenario
>> (esp., one of the main reasons sub-section hotadd of devmem was added).
>>
>> The issue is, that we have a valid memmap (pfn_valid()) for the
>> whole section, and the whole section will be marked "online".
>> pfn_to_online_page() will succeed, but the memmap contains garbage.
>>
>> E.g., doing a "cat /proc/kpageflags > /dev/null" results in
>>
>> [ 303.218313] BUG: unable to handle page fault for address: fffffffffffffffe
>> [ 303.218899] #PF: supervisor read access in kernel mode
>> [ 303.219344] #PF: error_code(0x0000) - not-present page
>> [ 303.219787] PGD 12614067 P4D 12614067 PUD 12616067 PMD 0
>> [ 303.220266] Oops: 0000 [#1] SMP NOPTI
>> [ 303.220587] CPU: 0 PID: 424 Comm: cat Not tainted 5.4.0-next-20191128+ #17
>

Hi Daniel,

> I can't reproduce this on x86-64 qemu, next-20191128 or mainline, with either
> memory size. What config are you using? How often are you hitting it?

Thanks for verifying! Hah, there is one piece missing to reproduce via
"cat /proc/kpageflags > /dev/null" that I ignored on my QEMU cmdline (see below)

I can reproduce it reliably (QEMU with "-m 4160M") via

[root@localhost ~]# uname -a
Linux localhost 5.5.0-rc1-next-20191209 #93 SMP Tue Dec 10 10:46:19 CET 2019 x86_64 x86_64 x86_64 GNU/Linux
[root@localhost ~]# ./page-types -r -a 0x144001
[ 200.476376] BUG: unable to handle page fault for address: fffffffffffffffe
[ 200.477500] #PF: supervisor read access in kernel mode
[ 200.478334] #PF: error_code(0x0000) - not-present page
[ 200.479076] PGD 59614067 P4D 59614067 PUD 59616067 PMD 0
[ 200.479557] Oops: 0000 [#4] SMP NOPTI
[ 200.479875] CPU: 0 PID: 603 Comm: page-types Tainted: G D W 5.5.0-rc1-next-20191209 #93
[ 200.480646] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu4
[ 200.481648] RIP: 0010:stable_page_flags+0x4d/0x410
[ 200.482061] Code: f3 ff 41 89 c0 48 b8 00 00 00 00 01 00 00 00 45 84 c0 0f 85 cd 02 00 00 48 8b 53 08 48 8b 2b 48f
[ 200.483644] RSP: 0018:ffffb139401cbe60 EFLAGS: 00010202
[ 200.484091] RAX: fffffffffffffffe RBX: fffffbeec5100040 RCX: 0000000000000000
[ 200.484697] RDX: 0000000000000001 RSI: ffffffff9535c7cd RDI: 0000000000000246
[ 200.485313] RBP: ffffffffffffffff R08: 0000000000000000 R09: 0000000000000000
[ 200.485917] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000144001
[ 200.486523] R13: 00007ffd6ba55f48 R14: 00007ffd6ba55f40 R15: ffffb139401cbf08
[ 200.487130] FS: 00007f68df717580(0000) GS:ffff9ec77fa00000(0000) knlGS:0000000000000000
[ 200.487804] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 200.488295] CR2: fffffffffffffffe CR3: 0000000135d48000 CR4: 00000000000006f0
[ 200.488897] Call Trace:
[ 200.489115] kpageflags_read+0xe9/0x140
[ 200.489447] proc_reg_read+0x3c/0x60
[ 200.489755] vfs_read+0xc2/0x170
[ 200.490037] ksys_pread64+0x65/0xa0
[ 200.490352] do_syscall_64+0x5c/0xa0
[ 200.490665] entry_SYSCALL_64_after_hwframe+0x49/0xbe

(tool located in tools/vm/page-types.c, see also patch #2)


To reproduce via "cat /proc/kpageflags > /dev/null", you have to
hot/coldplug one DIMM, to move max_pfn beyond the garbage memmap
(see also patch #2). My QEMU cmdline with Fedora 31:

qemu-system-x86_64 \
--enable-kvm \
-m 4160M,slots=4,maxmem=8G \
-hda Fedora-Cloud-Base-31-1.9.x86_64.qcow2 \
-machine pc \
-nographic \
-nodefaults \
-chardev stdio,id=serial,signal=off \
-device isa-serial,chardev=serial \
-object memory-backend-ram,id=mem0,size=1024M \
-device pc-dimm,id=dimm0,memdev=mem0

[root@localhost ~]# uname -a
Linux localhost 5.3.7-301.fc31.x86_64 #1 SMP Mon Oct 21 19:18:58 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
[root@localhost ~]# cat /proc/kpageflags > /dev/null
[ 111.517275] BUG: unable to handle page fault for address: fffffffffffffffe
[ 111.517907] #PF: supervisor read access in kernel mode
[ 111.518333] #PF: error_code(0x0000) - not-present page
[ 111.518771] PGD a240e067 P4D a240e067 PUD a2410067 PMD 0

>
> It may not have anything to do with the config, and I may be getting lucky with
> the garbage in my memory.
>

Some things that might be relevant from my config.

# CONFIG_PAGE_POISONING is not set
CONFIG_DEFERRED_STRUCT_PAGE_INIT=y
CONFIG_SPARSEMEM_EXTREME=y
CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
CONFIG_SPARSEMEM_VMEMMAP=y
CONFIG_HAVE_MEMBLOCK_NODE_MAP=y
CONFIG_MEMORY_HOTPLUG=y
CONFIG_MEMORY_HOTPLUG_SPARSE=y
CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y

The F31 default config should make it trigger.


Will update this patch description - thanks!

...

--
Thanks,

David / dhildenb

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