linux-kernel - Re: /proc/$pid/pagemap troubles

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Date:	Tue, 31 Jul 2007 16:06:21 -0700
From:	Dave Hansen <haveblue@...ibm.com>
To:	Andreas Schwab <schwab@...e.de>
Cc:	Matt Mackall <mpm@...enic.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: /proc/$pid/pagemap troubles

On Wed, 2007-08-01 at 00:58 +0200, Andreas Schwab wrote:
> Matt Mackall <mpm@...enic.com> writes:
> 
> > On Tue, Jul 31, 2007 at 01:36:14PM -0700, Dave Hansen wrote:
> >> Since the pagemap code has a little header on it to help describe the
> >> format, I wrote a little c program to parse its output.  I get some
> >> strange results.  If I do this:
> >> 
> >> 	fd = open("/proc/1/pagemap", O_RDONLY);
> >> 	count = read(fd, &endianness, 1);
> >> 
> >> count will always be 4.  
> >
> > Known bug, fixed in my pending and not-currently-working update. It
> > ought to return 0 for short reads.
> 
> That's not a good choice.  Returning 0 means EOF, but there is actually
> data to be read.

This should actually be pretty easy to fix.  We have a nice PAGE_SIZE
buffer.  So, if we are unaligned and would have overflowed the PAGE_SIZE
buffer, we return a short read.

If they ask for a <sizeof(unsigned long) read, we can copy that into the
PAGE_SIZE buffer, then just copy_to_user() the portion that was asked
for.  

-- Dave

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