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Re: kernel panic / NULL pointer dereference

To: linux-xfs@xxxxxxxxxxx
Subject: Re: kernel panic / NULL pointer dereference
From: Bernd Schubert <bernd.schubert@xxxxxxxxxxxxxxxxxx>
Date: Thu, 10 May 2012 18:43:10 +0200
Cc: "linux-fsdevel@xxxxxxxxxxxxxxx" <linux-fsdevel@xxxxxxxxxxxxxxx>
In-reply-to: <jognqm$o21$1@xxxxxxxxxxxxxxx>
References: <jognqm$o21$1@xxxxxxxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1
On 05/10/2012 05:45 PM, Bernd Schubert wrote:
> Hi all,
> 
> I'm just playing with an SRP connected NetApp system and just got an XFS 
> related kernel panic. I guess it is due to large IO (32MiB). At least it 
> just came up after enabling 32MiB device max_sectors.
> As the tests are running in a RHEL6 image and as I needed at least 
> 2.6.39 to get a large srp_tablsize with SRP, I simply installed the 
> lasted oracle uek kernel. If needed I'm going to update to a vanilla 
> version.
> 
> 
>> May 10 17:31:49 sgi01 kernel: XFS (sdb): Mounting Filesystem
>> May 10 17:31:49 sgi01 kernel: XFS (sdb): Ending clean mount
>> May 10 17:33:00 sgi01 kernel: BUG: unable to handle kernel NULL 
>> pointer dereference at (null)
>> May 10 17:33:00 sgi01 kernel: IP: [<ffffffffa07f5483>] 
>> xfs_alloc_ioend_bio+0x33/0x50 [xfs]

Oh, there is a bio allocation path to return NULL:

bvec_alloc_bs(gfp_mask, nr_iovecs, ) => NULL when nr_iovecs  > BIO_MAX_PAGES
bio_alloc_bioset(gfp_mask, nr_iovecs, ...)
bio_alloc(GFP_NOIO, nvecs)
xfs_alloc_ioend_bio()

And nvecs/nr_iovecs is obtained by bio_get_nr_vecs(), which does not check for
BIO_MAX_PAGES. Of course, all of that only happens with large IO sizes, 
which is exactly what I'm doing.
As xfs_alloc_ioend_bio() is using GFP_NOIO it does not expect bio_alloc 
to fail, but as I'm trying to send large IOs I guess that is exactly what 
happens here.


>May 10 17:33:00 sgi01 kernel: [<ffffffffa07f561e>] xfs_submit_ioend+0xfe/0x110 
>[xfs]
>May 10 17:33:00 sgi01 kernel: [<ffffffffa07f696b>] 
>xfs_vm_writepage+0x26b/0x510 [xfs]
>May 10 17:33:00 sgi01 kernel: [<ffffffff81112377>] __writepage+0x17/0x40
>May 10 17:33:00 sgi01 kernel: [<ffffffff81113696>] 
>write_cache_pages+0x246/0x520
>May 10 17:33:00 sgi01 kernel: [<ffffffff81112360>] ? set_page_dirty+0x70/0x70
>May 10 17:33:00 sgi01 kernel: [<ffffffff811139c1>] generic_writepages+0x51/0x80
>May 10 17:33:00 sgi01 kernel: [<ffffffffa07f537d>] xfs_vm_writepages+0x5d/0x80 
>[xfs]
>May 10 17:33:00 sgi01 kernel: [<ffffffff81113a11>] do_writepages+0x21/0x40
>May 10 17:33:00 sgi01 kernel: [<ffffffff8118df2e>] 
>writeback_single_inode+0x10e/0x270

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