linux-kernel - Re: [PATCH] Add a gfp-translate script to help understand page allocation failure reports

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Date:	Tue, 9 Jun 2009 09:05:10 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	riel@...hat.com, penberg@...helsinki.fi,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] Add a gfp-translate script to help understand page
	allocation failure reports

On Mon, Jun 08, 2009 at 04:38:27PM -0700, Andrew Morton wrote:
> On Mon, 8 Jun 2009 14:29:50 +0100
> Mel Gorman <mel@....ul.ie> wrote:
> 
> > The page allocation failure messages include a line that looks like
> > 
> > page allocation failure. order:1, mode:0x4020
> > 
> > The mode is easy to translate but irritating for the lazy and a bit error
> > prone. This patch adds a very simple helper script gfp-translate for the mode:
> > portion of the page allocation failure messages. An example usage looks like
> > 
> >   mel@...hina:~/linux-2.6 $ scripts/gfp-translate 0x4020
> >   Source: /home/mel/linux-2.6
> >   Parsing: 0x4020
> >   #define __GFP_HIGH	(0x20)	/* Should access emergency pools? */
> >   #define __GFP_COMP	(0x4000) /* Add compound page metadata */
> > 
> > The script is not a work of art but it has come in handy for me a few times
> > so I thought I would share.
> > 
> 
> hm, OK.  Most of the gfp masks I have to decode are in emails and
> bugzilla reports.  I guess I'm different. 

You can't be that different. Many of the ones I would be reading are from the
same places - mails (from lkml) and bugzilla (distros mainly). The minority
are ones I generated from my own tree and there I usually know what the
flags were anyway without the help of the script.

> Plus I wouldn't trust a tool
> run on my machine's kernel tree to correctly interpret a gfp mask from
> someone else's kernel of different vintage.
> 

There is an assumption that you can accurate recreate the reporters tree. If
that's wrong, you are in trouble anyway. Luckily, GFP flags are not that
changeable. If your copy of the kernel tree recognise the flag at all,
chances are it'll be interpreted correctly for the vast majority of flags. The
one possibly exception is __GFP_MOVABLE as there is a patch out there that
changes its definition.

> But I can see that it would be useful for someone who's debugging a
> locally built kernel.
> 
> > diff --git a/scripts/gfp-translate b/scripts/gfp-translate
> > new file mode 100755
> 
> I don't know how to get patches into Linus's tree with the X bit still
> set :(  To avoid solving that problem, maybe Pekka can merge this?
> 

Seems sensible. Pekka?

> > +# Guess the kernel source directory if it's not set. Preference is in order of
> > +# o current directory
> > +# o /usr/src/linux
> > +if [ "$SOURCE" = "" ]; then
> > +	if [ -r "/usr/src/linux/Makefile" ]; then
> > +		SOURCE=/usr/src/linux
> > +	fi
> > +	if [ -r "`pwd`/Makefile" ]; then
> > +		SOURCE=`pwd`
> > +	fi
> > +fi
> 
> OK.
> 
> > +# Confirm that a source directory exists
> > +if [ ! -r "$SOURCE/Makefile" ]; then
> > +	die "Could not locate source directory or it is invalid"
> > +fi
> 
> "kernel source directory".
> 

Sure. Thanks.

> > +# Confirm that a GFP mask has been specified
> > +if [ "$GFPMASK" = "none" ]; then
> > +	usage
> > +fi
> > +
> > +# Extract GFP flags from the kernel source
> > +TMPFILE=`mktemp -t gfptranslate-XXXXXX` || exit 1
> > +grep "^#define __GFP" $SOURCE/include/linux/gfp.h | sed -e 's/(__force gfp_t)//' | sed -e 's/u)/)/' | grep -v GFP_BITS | sed -e 's/)\//) \//' > $TMPFILE
> > +
> > +# Parse the flags
> > +IFS="
> > +"
> > +echo Source: $SOURCE
> > +echo Parsing: $GFPMASK
> > +for LINE in `cat $TMPFILE`; do
> > +	MASK=`echo $LINE | awk '{print $3}'`
> > +	if [ $(($GFPMASK&$MASK)) -ne 0 ]; then
> > +		echo $LINE
> > +	fi
> > +done
> > +
> > +rm -f $TMPFILE
> > +exit 0
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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