linux-kernel - Re: [BUG] Unable to handle kernel NULL pointer dereference...as_move_to

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Date:	Fri, 2 Feb 2007 12:56:30 -0800
From:	Andrew Vasquez <andrew.vasquez@...gic.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-SCSI Mailing List <linux-scsi@...r.kernel.org>,
	Nigel Kirkland <nigel.kirkland@...gic.com>
Subject: Re: [BUG] Unable to handle kernel NULL pointer dereference...as_move_to_dispatch+0x11/0x135

On Thu, 01 Feb 2007, Andrew Morton wrote:

> On Mon, 22 Jan 2007 10:35:10 -0800 Andrew Vasquez <andrew.vasquez@...gic.com> wrote:
> > Basically what is happening from the FC side is the initiator executes
> > a simple dt test:
> > 
> >         dt of=/dev/raw/raw1 procs=8 oncerr=abort bs=16k disable=stats limit=2m passes=1000000 pattern=iot dlimit=2048
> > 
> > against a single lun (a very basic Windows target mode driver).
> > During the test a port-enable, port-disable script is running agains
> > the switch's port that is connected to the target (this occurs every
> > sixty seconds (for a disabled duration of 2 seconds).  Additionally,
> > the target itself is set to LOGO (logout) or drop off the topology
> > every 30 seconds.
> 
> I don't understand what effect the port-enable/port-disable has upon the
> system.  Will it cause I/O errors, or what?

No I/O errors should make there way to the upper-layers (block/FS).
The system *should* be shielded from the fibre-channel fabric events.
I just wanted to explain what the (basic sanity) test did.

> > This test runs fine up to 2.6.19.
> 
> One thing we did in there was to give direct-io-against-blockdevs some
> special-case bio-preparation code.  Perhaps this is tickling a bug somehow.
> 
> We can revert that change like this:
> 
> 
> diff -puN fs/block_dev.c~a fs/block_dev.c
> --- a/fs/block_dev.c~a
> +++ a/fs/block_dev.c
> @@ -196,8 +196,47 @@ static void blk_unget_page(struct page *
>  	pvec->page[--pvec->idx] = page;
>  }
>  
> +static int
> +blkdev_get_blocks(struct inode *inode, sector_t iblock,
> +		struct buffer_head *bh, int create)
...

Hmm, with this patch we've noted two main differences:

1) I/O throughput with the basic 'dd' command used (above) is back to
   60MB/s, rather than the appalling 20-22 MB/s we were seeing with
   2.6.20-rcX.

2) No panics -- so far with 2+ hours of testing.  With our vanilla
   system of 2.6.20-rc7, the test could trigger the panic within 15 to
   20 minutes.

We'll let this run over the weekend -- I'll certainly let you know if
anything has changed (failures).

--
Andrew Vasquez
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