Re: [PATCH v1] mm: zero remaining unavailable struct pages (Re: kernel panic in reading /proc/kpageflags when enabling RAM-simulated PMEM)

[Michal Hocko]

On Wed 13-06-18 05:41:08, Naoya Horiguchi wrote:
[...]
> From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Date: Wed, 13 Jun 2018 12:43:27 +0900
> Subject: [PATCH] mm: zero remaining unavailable struct pages
> 
> There is a kernel panic that is triggered when reading /proc/kpageflags
> on the kernel booted with kernel parameter 'memmap=nn[KMG]!ss[KMG]':
> 
>   BUG: unable to handle kernel paging request at fffffffffffffffe
>   PGD 9b20e067 P4D 9b20e067 PUD 9b210067 PMD 0
>   Oops: 0000 [#1] SMP PTI
>   CPU: 2 PID: 1728 Comm: page-types Not tainted 4.17.0-rc6-mm1-v4.17-rc6-180605-0816-00236-g2dfb086ef02c+ #160
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.fc28 04/01/2014
>   RIP: 0010:stable_page_flags+0x27/0x3c0
>   Code: 00 00 00 0f 1f 44 00 00 48 85 ff 0f 84 a0 03 00 00 41 54 55 49 89 fc 53 48 8b 57 08 48 8b 2f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 01 0f 84 10 03 00 00 31 db 49 8b 54 24 08 4c 89 e7
>   RSP: 0018:ffffbbd44111fde0 EFLAGS: 00010202
>   RAX: fffffffffffffffe RBX: 00007fffffffeff9 RCX: 0000000000000000
>   RDX: 0000000000000001 RSI: 0000000000000202 RDI: ffffed1182fff5c0
>   RBP: ffffffffffffffff R08: 0000000000000001 R09: 0000000000000001
>   R10: ffffbbd44111fed8 R11: 0000000000000000 R12: ffffed1182fff5c0
>   R13: 00000000000bffd7 R14: 0000000002fff5c0 R15: ffffbbd44111ff10
>   FS:  00007efc4335a500(0000) GS:ffff93a5bfc00000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: fffffffffffffffe CR3: 00000000b2a58000 CR4: 00000000001406e0
>   Call Trace:
>    kpageflags_read+0xc7/0x120
>    proc_reg_read+0x3c/0x60
>    __vfs_read+0x36/0x170
>    vfs_read+0x89/0x130
>    ksys_pread64+0x71/0x90
>    do_syscall_64+0x5b/0x160
>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
>   RIP: 0033:0x7efc42e75e23
>   Code: 09 00 ba 9f 01 00 00 e8 ab 81 f4 ff 66 2e 0f 1f 84 00 00 00 00 00 90 83 3d 29 0a 2d 00 00 75 13 49 89 ca b8 11 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 34 c3 48 83 ec 08 e8 db d3 01 00 48 89 04 24
> 
> According to kernel bisection, this problem became visible due to commit
> f7f99100d8d9 which changes how struct pages are initialized.
> 
> Memblock layout affects the pfn ranges covered by node/zone. Consider
> that we have a VM with 2 NUMA nodes and each node has 4GB memory, and
> the default (no memmap= given) memblock layout is like below:
> 
>   MEMBLOCK configuration:
>    memory size = 0x00000001fff75c00 reserved size = 0x000000000300c000
>    memory.cnt  = 0x4
>    memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
>    memory[0x1]     [0x0000000000100000-0x00000000bffd6fff], 0x00000000bfed7000 bytes on node 0 flags: 0x0
>    memory[0x2]     [0x0000000100000000-0x000000013fffffff], 0x0000000040000000 bytes on node 0 flags: 0x0
>    memory[0x3]     [0x0000000140000000-0x000000023fffffff], 0x0000000100000000 bytes on node 1 flags: 0x0
>    ...
> 
> If you give memmap=1G!4G (so it just covers memory[0x2]),
> the range [0x100000000-0x13fffffff] is gone:
> 
>   MEMBLOCK configuration:
>    memory size = 0x00000001bff75c00 reserved size = 0x000000000300c000
>    memory.cnt  = 0x3
>    memory[0x0]     [0x0000000000001000-0x000000000009efff], 0x000000000009e000 bytes on node 0 flags: 0x0
>    memory[0x1]     [0x0000000000100000-0x00000000bffd6fff], 0x00000000bfed7000 bytes on node 0 flags: 0x0
>    memory[0x2]     [0x0000000140000000-0x000000023fffffff], 0x0000000100000000 bytes on node 1 flags: 0x0
>    ...
> 
> This causes shrinking node 0's pfn range because it is calculated by
> the address range of memblock.memory. So some of struct pages in the
> gap range are left uninitialized.
> 
> We have a function zero_resv_unavail() which does zeroing the struct
> pages outside memblock.memory, but currently it covers only the reserved
> unavailable range (i.e. memblock.memory && !memblock.reserved).
> This patch extends it to cover all unavailable range, which fixes
> the reported issue.

Thanks for pin pointing this down Naoya! I am wondering why we cannot
simply mark the excluded ranges to be reserved instead.
-- 
Michal Hocko
SUSE Labs