Re: [PATCH V2] mm: check pfn_valid first in zero_resv_unavail

[Michal Hocko <mhocko@kernel.org>]

On Fri 01-12-17 17:50:48, Dave Young wrote:
> With latest kernel I get below bug while testing kdump:
> [    0.000000] BUG: unable to handle kernel paging request at ffffea00034b1040
> [    0.000000] IP: zero_resv_unavail+0xbd/0x126
> [    0.000000] PGD 37b98067 P4D 37b98067 PUD 37b97067 PMD 0 
> [    0.000000] Oops: 0002 [#1] SMP
> [    0.000000] Modules linked in:
> [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.15.0-rc1+ #316
> [    0.000000] Hardware name: LENOVO 20ARS1BJ02/20ARS1BJ02, BIOS GJET92WW (2.42 ) 03/03/2017
> [    0.000000] task: ffffffff81a0e4c0 task.stack: ffffffff81a00000
> [    0.000000] RIP: 0010:zero_resv_unavail+0xbd/0x126
> [    0.000000] RSP: 0000:ffffffff81a03d88 EFLAGS: 00010006
> [    0.000000] RAX: 0000000000000000 RBX: ffffea00034b1040 RCX: 0000000000000010
> [    0.000000] RDX: 0000000000000000 RSI: 0000000000000092 RDI: ffffea00034b1040
> [    0.000000] RBP: 00000000000d2c41 R08: 00000000000000c0 R09: 0000000000000a0d
> [    0.000000] R10: 0000000000000002 R11: 0000000000007f01 R12: ffffffff81a03d90
> [    0.000000] R13: ffffea0000000000 R14: 0000000000000063 R15: 0000000000000062
> [    0.000000] FS:  0000000000000000(0000) GS:ffffffff81c73000(0000) knlGS:0000000000000000
> [    0.000000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    0.000000] CR2: ffffea00034b1040 CR3: 0000000037609000 CR4: 00000000000606b0
> [    0.000000] Call Trace:
> [    0.000000]  ? free_area_init_nodes+0x640/0x664
> [    0.000000]  ? zone_sizes_init+0x58/0x72
> [    0.000000]  ? setup_arch+0xb50/0xc6c
> [    0.000000]  ? start_kernel+0x64/0x43d
> [    0.000000]  ? secondary_startup_64+0xa5/0xb0
> [    0.000000] Code: c1 e8 0c 48 39 d8 76 27 48 89 de 48 c1 e3 06 48 c7 c7 7a 87 79 81 e8 b0 c0 3e ff 4c 01 eb b9 10 00 00 00 31 c0 48 89 df 49 ff c6 <f3> ab eb bc 6a 00 49 
> c7 c0 f0 93 d1 81 31 d2 83 ce ff 41 54 49 
> [    0.000000] RIP: zero_resv_unavail+0xbd/0x126 RSP: ffffffff81a03d88
> [    0.000000] CR2: ffffea00034b1040
> [    0.000000] ---[ end trace f5ba9e8f73c7ee26 ]---
> 
> This is introduced with commit a4a3ede2132a ("mm: zero reserved and
> unavailable struct pages")
> 
> The reason is some efi reserved boot ranges is not reported in E820 ram.
> In my case it is a bgrt buffer:
> efi: mem00: [Boot Data          |RUN|  |  |  |  |  |  |   |WB|WT|WC|UC] range=[0x00000000d2c41000-0x00000000d2c85fff] (0MB)

I am still confused. Could you clarify why does efi code reserve this
range when it is not backed by any real memory?
-- 
Michal Hocko
SUSE Labs

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