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

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

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@ah.jp.nec.com>
> > 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: */


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.

Thanks,
Naoya Horiguchi