Re: [PATCH 2/2] /proc/kpageflags: do not use uninitialized struct pages

[Toshiki Fukasawa]

On 2019/07/26 16:06, Michal Hocko wrote:
> On Fri 26-07-19 06:25:49, Toshiki Fukasawa wrote:
>>
>>
>> On 2019/07/25 18:03, Michal Hocko wrote:
>>> On Thu 25-07-19 02:31:18, Toshiki Fukasawa wrote:
>>>> A kernel panic was observed during reading /proc/kpageflags for
>>>> first few pfns allocated by pmem namespace:
>>>>
>>>> BUG: unable to handle page fault for address: fffffffffffffffe
>>>> [  114.495280] #PF: supervisor read access in kernel mode
>>>> [  114.495738] #PF: error_code(0x0000) - not-present page
>>>> [  114.496203] PGD 17120e067 P4D 17120e067 PUD 171210067 PMD 0
>>>> [  114.496713] Oops: 0000 [#1] SMP PTI
>>>> [  114.497037] CPU: 9 PID: 1202 Comm: page-types Not tainted 5.3.0-rc1 #1
>>>> [  114.497621] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
>>>> [  114.498706] RIP: 0010:stable_page_flags+0x27/0x3f0
>>>> [  114.499142] Code: 82 66 90 66 66 66 66 90 48 85 ff 0f 84 d1 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 0f 84 57 03 00 00 45 31 e4 48 8b 55 08 48 89 ef
>>>> [  114.500788] RSP: 0018:ffffa5e601a0fe60 EFLAGS: 00010202
>>>> [  114.501373] RAX: fffffffffffffffe RBX: ffffffffffffffff RCX: 0000000000000000
>>>> [  114.502009] RDX: 0000000000000001 RSI: 00007ffca13a7310 RDI: ffffd07489000000
>>>> [  114.502637] RBP: ffffd07489000000 R08: 0000000000000001 R09: 0000000000000000
>>>> [  114.503270] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000240000
>>>> [  114.503896] R13: 0000000000080000 R14: 00007ffca13a7310 R15: ffffa5e601a0ff08
>>>> [  114.504530] FS:  00007f0266c7f540(0000) GS:ffff962dbbac0000(0000) knlGS:0000000000000000
>>>> [  114.505245] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> [  114.505754] CR2: fffffffffffffffe CR3: 000000023a204000 CR4: 00000000000006e0
>>>> [  114.506401] Call Trace:
>>>> [  114.506660]  kpageflags_read+0xb1/0x130
>>>> [  114.507051]  proc_reg_read+0x39/0x60
>>>> [  114.507387]  vfs_read+0x8a/0x140
>>>> [  114.507686]  ksys_pread64+0x61/0xa0
>>>> [  114.508021]  do_syscall_64+0x5f/0x1a0
>>>> [  114.508372]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>>> [  114.508844] RIP: 0033:0x7f0266ba426b
>>>>
>>>> The reason for the panic is that stable_page_flags() which parses
>>>> the page flags uses uninitialized struct pages reserved by the
>>>> ZONE_DEVICE driver.
>>>
>>> Why pmem hasn't initialized struct pages?
>>
>> We proposed to initialize in previous approach but that wasn't merged.
>> (See https://marc.info/?l=linux-mm&m=152964792500739&w=2)
>>
>>> Isn't that a bug that should be addressed rather than paper over it like this?
>>
>> I'm not sure. What do you think, Dan?
> 
> Yeah, I am really curious about details. Why do we keep uninitialized
> struct pages at all? What is a random pfn walker supposed to do? What
> kind of metadata would be clobbered? In other words much more details
> please.
> 
I also want to know. I do not think that initializing struct pages will
clobber any metadata.

Best regards,
Toshiki Fukasawa