linux-kernel - Re: [PATCH v8 04/22] Change direct

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Message-ID: <20140731141315.GT6754@linux.intel.com>
Date:	Thu, 31 Jul 2014 10:13:15 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Boaz Harrosh <openosd@...il.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 04/22] Change direct_access calling convention

On Thu, Jul 31, 2014 at 01:11:42PM +0300, Boaz Harrosh wrote:
> >>> +	if (size < 0)
> >>
> >> 	if(size < PAGE_SIZE), No?
> > 
> > No, absolutely not.  PAGE_SIZE is unsigned long, which (if I understand
> > my C integer promotions correctly) means that 'size' gets promoted to
> > an unsigned long, and we compare them unsigned, so errors will never be
> > caught by this check.
> 
> Good point I agree that you need a cast ie.
> 
>  	if(size < (long)PAGE_SIZE)
> 
> The reason I'm saying this is because of a bug I actually hit when
> playing with partitioning and fdisk, it came out that the last partition's
> size was not page aligned, and code that checked for (< 0) crashed because
> prd returned the last two sectors of the partition, since your API is sector
> based this can happen for you here, before you are memseting a PAGE_SIZE
> you need to test there is space, No? 

Not in ext2/ext4.  It requires block size == PAGE_SIZE, so it's never
going to request the last partial block in a partition.
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