linux-kernel - Re: Cannot partition 32GB disk on a 32bit machine (correct version of the patch this time)

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Message-ID: <1403203437.17294.25.camel@dabdike.int.hansenpartnership.com>
Date:	Thu, 19 Jun 2014 11:43:57 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	"Cox, Alan" <alan.cox@...el.com>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"aeb@....nl" <aeb@....nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Cannot partition 32GB disk on a 32bit machine (correct version
 of the patch this time)

On Thu, 2014-06-19 at 09:33 +0000, Cox, Alan wrote:
> On Thu, 2014-06-19 at 10:30 +0100, Alan Cox wrote:
> > The block code has 32bit cleanness problems with the iterator. This
> > prevents things like partitioning a 32GB volume on a 32bit system.
> > 
> > I hit this with a volume of exactly 32GB in size (easy to duplicate with
> > virtual machines). Tracing at step by step through the kernel I found
> > the problem lines in blkdev_read_iter which truncates the size value
> > into a 32bit value when setting up the iterator.
> > 
> > The hack below if applied "fixes" this and I think demonstrates that
> > this is the problem spot.
> > 
> > What I'm less clear on is what the correct fix for this should be.
> 
> block: fix inability to partition large volumes
> 
> From: Alan <alan@...ux.intel.com>
> 
> The block code has 32bit cleanness problems with the iterator. This
> prevents things like partitioning a 32GB volume on a 32bit system.
> 
> This is a simple initial "fix" that clips the problem cases so get
> behaviour that is at least sane and trivially backportable.
> 
> Signed-off-by: Alan Cox <alan@...ux.intel.com>
> ---
>  fs/block_dev.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 6d72746..bef2414 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1603,6 +1603,9 @@ static ssize_t blkdev_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
>  
>  	size -= pos;
>  	iov_iter_truncate(to, size);
> +	/* Fix up for 32bit boxes for now */
> +	if (to->count < size)
> +	        to->count = 0xFFFFFFFF;
>  	return generic_file_read_iter(iocb, to);
>  }
>  
> 
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Wow that's junk issued by an Exchange server ... Alan, really ...

Do you have CONFIG_LBD turned on?  That's supposed to let us go up to
about 16TB before we run out of page index bits.  If you do, we might
have a variable that's int but should be sector_t somewhere.

James


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