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

From: Michal Hocko
Date: Thu Jun 14 2018 - 03:00:57 EST

On Thu 14-06-18 05:16:18, Naoya Horiguchi wrote:
> On Wed, Jun 13, 2018 at 11:07:00AM +0200, Michal Hocko wrote:
> > 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.
>
> I tried your idea with the change below, and it also fixes the kernel panic.
>
> ---
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index d1f25c831447..2cef120535d4 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -1248,6 +1248,7 @@ void __init e820__memblock_setup(void)
> {
> int i;
> u64 end;
> + u64 addr = 0;
>
> /*
> * The bootstrap memblock region count maximum is 128 entries
> @@ -1264,13 +1265,16 @@ void __init e820__memblock_setup(void)
> struct e820_entry *entry = &e820_table->entries[i];
>
> end = entry->addr + entry->size;
> + if (addr < entry->addr)
> + memblock_reserve(addr, entry->addr - addr);
> + addr = end;
> if (end != (resource_size_t)end)
> continue;
>
> if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
> - continue;
> -
> - memblock_add(entry->addr, entry->size);
> + memblock_reserve(entry->addr, entry->size);
> + else
> + memblock_add(entry->addr, entry->size);
> }
>
> /* Throw away partial pages: */

Yes, this looks so much better. Although I was more focusing on
e820__range_remove.

> My concern is that there are a few E820 memory types rather than
> E820_TYPE_RAM and E820_TYPE_RESERVED, and I'm not sure that putting them
> all into memblock.reserved is really acceptable.

Why it wouldn't? I mean reserved ranges are to be never touched unless
the owner of that range free them up.
--
Michal Hocko
SUSE Labs