Re: [BUG] kernel BUG at drivers/block/xen-blkfront.c:1711

[Evgenii Shatokhin <eshatokhin@virtuozzo.com>]

On 11.07.2016 15:04, Bob Liu wrote:
>
>
> On 07/11/2016 04:50 PM, Evgenii Shatokhin wrote:
>> On 06.06.2016 11:42, Dario Faggioli wrote:
>>> Just Cc-ing some Linux, block, and Xen on CentOS people...
>>>
>>
>> Ping.
>>
>> Any suggestions how to debug this or what might cause the problem?
>>
>> Obviously, we cannot control Xen on the Amazon's servers. But perhaps there is something we can do at the kernel's side, is it?
>>
>>> On Mon, 2016-06-06 at 11:24 +0300, Evgenii Shatokhin wrote:
>>>> (Resending this bug report because the message I sent last week did
>>>> not
>>>> make it to the mailing list somehow.)
>>>>
>>>> Hi,
>>>>
>>>> One of our users gets kernel panics from time to time when he tries
>>>> to
>>>> use his Amazon EC2 instance with CentOS7 x64 in it [1]. Kernel panic
>>>> happens within minutes from the moment the instance starts. The
>>>> problem
>>>> does not show up every time, however.
>>>>
>>>> The user first observed the problem with a custom kernel, but it was
>>>> found later that the stock kernel 3.10.0-327.18.2.el7.x86_64 from
>>>> CentOS7 was affected as well.
>
> Please try this patch:
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7b0767502b5db11cb1f0daef2d01f6d71b1192dc
>
> Regards,
> Bob
>

Unfortunately, it did not help. The same BUG_ON() in 
blkfront_setup_indirect() still triggers in our kernel based on RHEL's 
3.10.0-327.18.2, where I added the patch.

As far as I can see, the patch makes sure the indirect pages are added 
to the list only if (!info->feature_persistent) holds. I suppose it 
holds in our case and the pages are added to the list because the 
triggered BUG_ON() is here:

	if (!info->feature_persistent && info->max_indirect_segments) {
		<...>
		BUG_ON(!list_empty(&info->indirect_pages));
		<...>
	}

So the problem is still out there somewhere, it seems.

Regards,
Evgenii

>>>>
>>>> The part of the system log he was able to retrieve is attached. Here
>>>> is
>>>> the bug info, for convenience:
>>>>
>>>> ------------------------------------
>>>> [    2.246912] kernel BUG at drivers/block/xen-blkfront.c:1711!
>>>> [    2.246912] invalid opcode: 0000 [#1] SMP
>>>> [    2.246912] Modules linked in: ata_generic pata_acpi
>>>> crct10dif_pclmul
>>>> crct10dif_common crc32_pclmul crc32c_intel ghash_clmulni_intel
>>>> xen_netfront xen_blkfront(+) aesni_intel lrw ata_piix gf128mul
>>>> glue_helper ablk_helper cryptd libata serio_raw floppy sunrpc
>>>> dm_mirror
>>>> dm_region_hash dm_log dm_mod scsi_transport_iscsi
>>>> [    2.246912] CPU: 1 PID: 50 Comm: xenwatch Not tainted
>>>> 3.10.0-327.18.2.el7.x86_64 #1
>>>> [    2.246912] Hardware name: Xen HVM domU, BIOS 4.2.amazon
>>>> 12/07/2015
>>>> [    2.246912] task: ffff8800e9fcb980 ti: ffff8800e98bc000 task.ti:
>>>> ffff8800e98bc000
>>>> [    2.246912] RIP: 0010:[<ffffffffa015584f>]  [<ffffffffa015584f>]
>>>> blkfront_setup_indirect+0x41f/0x430 [xen_blkfront]
>>>> [    2.246912] RSP: 0018:ffff8800e98bfcd0  EFLAGS: 00010283
>>>> [    2.246912] RAX: ffff8800353e15c0 RBX: ffff8800e98c52c8 RCX:
>>>> 0000000000000020
>>>> [    2.246912] RDX: ffff8800353e15b0 RSI: ffff8800e98c52b8 RDI:
>>>> ffff8800353e15d0
>>>> [    2.246912] RBP: ffff8800e98bfd20 R08: ffff8800353e15b0 R09:
>>>> ffff8800eb403c00
>>>> [    2.246912] R10: ffffffffa0155532 R11: ffffffffffffffe8 R12:
>>>> ffff8800e98c4000
>>>> [    2.246912] R13: ffff8800e98c52b8 R14: 0000000000000020 R15:
>>>> ffff8800353e15c0
>>>> [    2.246912] FS:  0000000000000000(0000) GS:ffff8800efc20000(0000)
>>>> knlGS:0000000000000000
>>>> [    2.246912] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> [    2.246912] CR2: 00007f1b615ef000 CR3: 00000000e2b44000 CR4:
>>>> 00000000001406e0
>>>> [    2.246912] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
>>>> 0000000000000000
>>>> [    2.246912] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
>>>> 0000000000000400
>>>> [    2.246912] Stack:
>>>> [    2.246912]  0000000000000020 0000000000000001 00000020a0157217
>>>> 00000100e98bfdbc
>>>> [    2.246912]  0000000027efa3ef ffff8800e98bfdbc ffff8800e98ce000
>>>> ffff8800e98c4000
>>>> [    2.246912]  ffff8800e98ce040 0000000000000001 ffff8800e98bfe08
>>>> ffffffffa0155d4c
>>>> [    2.246912] Call Trace:
>>>> [    2.246912]  [<ffffffffa0155d4c>] blkback_changed+0x4ec/0xfc8
>>>> [xen_blkfront]
>>>> [    2.246912]  [<ffffffff813a6fd0>] ? xenbus_gather+0x170/0x190
>>>> [    2.246912]  [<ffffffff816322f5>] ? __slab_free+0x10e/0x277
>>>> [    2.246912]  [<ffffffff813a805d>]
>>>> xenbus_otherend_changed+0xad/0x110
>>>> [    2.246912]  [<ffffffff813a7257>] ? xenwatch_thread+0x77/0x180
>>>> [    2.246912]  [<ffffffff813a9ba3>] backend_changed+0x13/0x20
>>>> [    2.246912]  [<ffffffff813a7246>] xenwatch_thread+0x66/0x180
>>>> [    2.246912]  [<ffffffff810a6ae0>] ? wake_up_atomic_t+0x30/0x30
>>>> [    2.246912]  [<ffffffff813a71e0>] ?
>>>> unregister_xenbus_watch+0x1f0/0x1f0
>>>> [    2.246912]  [<ffffffff810a5aef>] kthread+0xcf/0xe0
>>>> [    2.246912]  [<ffffffff810a5a20>] ?
>>>> kthread_create_on_node+0x140/0x140
>>>> [    2.246912]  [<ffffffff81646118>] ret_from_fork+0x58/0x90
>>>> [    2.246912]  [<ffffffff810a5a20>] ?
>>>> kthread_create_on_node+0x140/0x140
>>>> [    2.246912] Code: e1 48 85 c0 75 ce 49 8d 84 24 40 01 00 00 48 89
>>>> 45
>>>> b8 e9 91 fd ff ff 4c 89 ff e8 8d ae 06 e1 e9 f2 fc ff ff 31 c0 e9 2e
>>>> fe
>>>> ff ff <0f> 0b e8 9a 57 f2 e0 0f 0b 0f 1f 84 00 00 00 00 00 0f 1f 44
>>>> 00
>>>> [    2.246912] RIP  [<ffffffffa015584f>]
>>>> blkfront_setup_indirect+0x41f/0x430 [xen_blkfront]
>>>> [    2.246912]  RSP <ffff8800e98bfcd0>
>>>> [    2.491574] ---[ end trace 8a9b992812627c71 ]---
>>>> [    2.495618] Kernel panic - not syncing: Fatal exception
>>>> ------------------------------------
>>>>
>>>> Xen version 4.2.
>>>>
>>>> EC2 instance type: c3.large with EBS magnetic storage, if that
>>>> matters.
>>>>
>>>> Here is the code where the BUG_ON triggers (drivers/block/xen-
>>>> blkfront.c):
>>>> ------------------------------------
>>>> if (!info->feature_persistent && info->max_indirect_segments) {
>>>>        /*
>>>>            * We are using indirect descriptors but not persistent
>>>>            * grants, we need to allocate a set of pages that can be
>>>>            * used for mapping indirect grefs
>>>>            */
>>>>        int num = INDIRECT_GREFS(segs) * BLK_RING_SIZE;
>>>>
>>>>        BUG_ON(!list_empty(&info->indirect_pages)); // << This one hits.
>>>>        for (i = 0; i < num; i++) {
>>>>            struct page *indirect_page = alloc_page(GFP_NOIO);
>>>>            if (!indirect_page)
>>>>                goto out_of_memory;
>>>>            list_add(&indirect_page->lru, &info->indirect_pages);
>>>>        }
>>>> }
>>>> ------------------------------------
>>>>
>>>> As we checked, 'info->indirect_pages' list indeed contained around
>>>> 30
>>>> elements at that point.
>>>>
>>>> Any ideas what may cause this and how to fix it?
>>>>
>>>> If any other data are needed, please let me know.
>>>>
>>>> References:
>>>> [1] https://bugs.openvz.org/browse/OVZ-6718
>
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