Re: "mm: use early_pfn_to_nid in page_ext_init" broken on some configurations?

[Vlastimil Babka]

On 06/30/2017 04:18 PM, Michal Hocko wrote:
> fe53ca54270a ("mm: use early_pfn_to_nid in page_ext_init") seem
> to silently depend on CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID resp.
> CONFIG_HAVE_MEMBLOCK_NODE_MAP. early_pfn_to_nid is returning zero with
> !defined(CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID) && !defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP)
> I am not sure how widely is this used but such a code is tricky. I see
> how catching early allocations during defered initialization might be
> useful but a subtly broken code sounds like a problem to me.  So is
> fe53ca54270a worth this or we should revert it?

There might be more issues with fe53ca54270a, I think. This I've
observed on our 4.4-based kernel, which has deferred page struct init,
but doesn't have b8f1a75d61d8 ("mm: call page_ext_init() after all
struct pages are initialized") nor aforementioned fe53ca54270a:

[    0.000000] allocated 421003264 bytes of page_ext
[    0.000000] Node 0, zone      DMA: page owner found early allocated 0 pages
[    0.000000] Node 0, zone    DMA32: page owner found early allocated 33 pages
[    0.000000] Node 0, zone   Normal: page owner found early allocated 2842622 pages
[    0.000000] BUG: unable to handle kernel NULL pointer dereference at           (null)
[    0.000000] IP: [<ffffffff811f090a>] init_page_owner+0x12a/0x240
[    0.000000] PGD 0 
[    0.000000] Oops: 0000 [#1] SMP 
[    0.000000] Modules linked in:
[    0.000000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.4.74+ #7
[    0.000000] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
[    0.000000] task: ffffffff81e104c0 ti: ffffffff81e00000 task.ti: ffffffff81e00000
[    0.000000] RIP: 0010:[<ffffffff811f090a>]  [<ffffffff811f090a>] init_page_owner+0x12a/0x240
[    0.000000] RSP: 0000:ffffffff81e03ed0  EFLAGS: 00010046
[    0.000000] RAX: 0000000000000000 RBX: ffff88083ffe0210 RCX: ffffea0013000000
[    0.000000] RDX: 0000000000000300 RSI: ffffffff81f57437 RDI: 00000000004c0000
[    0.000000] RBP: ffffffff81e03f20 R08: ffffffff81e03e90 R09: 0000000000000000
[    0.000000] R10: 00000000004c0200 R11: 0000000000000000 R12: ffffea0000000000
[    0.000000] R13: 00000000004c0200 R14: 00000000004c0000 R15: 0000000000840000
[    0.000000] FS:  0000000000000000(0000) GS:ffff88042fc00000(0000) knlGS:0000000000000000
[    0.000000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.000000] CR2: 0000000000000000 CR3: 0000000001e0b000 CR4: 00000000000406b0
[    0.000000] Stack:
[    0.000000]  0000000000000206 ffff88083ffe0f90 ffff88083ffdf000 0000000000003181
[    0.000000]  ffffea0013000000 0000000000000040 ffffea0000000000 0000000000840000
[    0.000000]  0000000000840000 000000008e000010 ffffffff81e03f50 ffffffff81f84145
[    0.000000] Call Trace:
[    0.000000]  [<ffffffff81f84145>] page_ext_init+0x15e/0x167
[    0.000000]  [<ffffffff81f57e6a>] start_kernel+0x351/0x418
[    0.000000]  [<ffffffff81f57120>] ? early_idt_handler_array+0x120/0x120
[    0.000000]  [<ffffffff81f57309>] x86_64_start_reservations+0x2a/0x2c
[    0.000000]  [<ffffffff81f57437>] x86_64_start_kernel+0x12c/0x13b
[    0.000000] Code: 81 e2 00 fe ff ff 4d 39 fa 4d 0f 47 d7 4d 39 f2 4d 89 d5 77 34 eb 5e 48 8b 01 f6 c4 04 75 21 48 89 cf 48 89 4d d0 e8 b6 35 00 00 <48> 8b 00 a8 04 75 0e 48 8b 4d d0 e9 c2 00 00 00 48 83 45 c8 01 
[    0.000000] RIP  [<ffffffff811f090a>] init_page_owner+0x12a/0x240
[    0.000000]  RSP <ffffffff81e03ed0>
[    0.000000] CR2: 0000000000000000
[    0.000000] ---[ end trace 19e05592f03a690f ]---

Note that this is different backtrace than in b8f1a75d61d8 log.

Still, backporting b8f1a75d61d8 fixes this:

[    1.538379] allocated 738197504 bytes of page_ext
[    1.539340] Node 0, zone      DMA: page owner found early allocated 0 pages
[    1.540179] Node 0, zone    DMA32: page owner found early allocated 33 pages
[    1.611173] Node 0, zone   Normal: page owner found early allocated 96755 pages
[    1.683167] Node 1, zone   Normal: page owner found early allocated 96575 pages

No panic, notice how it allocated more for page_ext, and found smaller number of
early allocated pages.

Now backporting fe53ca54270a on top:

[    0.000000] allocated 738197504 bytes of page_ext
[    0.000000] Node 0, zone      DMA: page owner found early allocated 0 pages
[    0.000000] Node 0, zone    DMA32: page owner found early allocated 33 pages
[    0.000000] Node 0, zone   Normal: page owner found early allocated 2842622 pages
[    0.000000] Node 1, zone   Normal: page owner found early allocated 3694362 pages

Again no panic, and same amount of page_ext usage. But the "early allocated" numbers
seem bogus to me. I think it's because init_pages_in_zone() is running and inspecting
struct pages that have not been yet initialized. It doesn't end up crashing, but
still doesn't seem correct?