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SubjectRe: [PATCH v1] mm: zero remaining unavailable struct pages (Re: kernel panic in reading /proc/kpageflags when enabling RAM-simulated PMEM)
On Thu, Jun 14, 2018 at 05:16:18AM +0000, 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@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.

Hi Naoya,

Maybe you could just add to memblock.reserved, all unavailable ranges within
E820_TYPE_RAM.
Actually, in your original patch, you are walking memblock.memory, which should
only contain E820_TYPE_RAM ranges (talking about x86).

So I think the below would to the trick as well?

@@ -1248,6 +1276,7 @@ void __init e820__memblock_setup(void)
{
int i;
u64 end;
+ u64 next = 0;

/*
* The bootstrap memblock region count maximum is 128 entries
@@ -1269,6 +1299,14 @@ void __init e820__memblock_setup(void)

if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
continue;
+
+ if (entry->type == E820_TYPE_RAM)
+ if (next < entry->addr) {
+ memblock_reserve (next, next + (entry->addr - next));
+ next = end;
+ }
With the above patch, I can no longer see the issues either.

Although, there is a difference between this and your original patch.
In your original patch, you are just zeroing the pages, while with this one (or with your second patch),
we will zero the page in reserve_bootmem_region(), but that function also init
some other fields of the struct page:

mm_zero_struct_page(page);
set_page_links(page, zone, nid, pfn);
init_page_count(page);
page_mapcount_reset(page);
page_cpupid_reset_last(page);

So I am not sure we want to bother doing that for pages that are really unreachable.

Oscar Salvador
Best Regards

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