linux-kernel - Re: [PATCH] x86: endless page faults in mount_block

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Date:	Wed, 30 Apr 2008 00:24:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Henry Nestler <Henry.Ne@...or.de>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alexander Viro <viro@....linux.org.uk>,
	Vegard Nossum <vegard.nossum@...il.com>
Subject: Re: [PATCH] x86: endless page faults in mount_block_root for Linux
	2.6


* Pekka Enberg <penberg@...helsinki.fi> wrote:

> On Tue, Apr 29, 2008 at 5:33 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >  btw., i have a kmemcheck-reported bug fixed in this same area with the
> >  patch below. I dont remember the details anymore, but the root mount
> >  code did something really, really weird here.
> >
> >  Subject: init: root mount fix
> >  From: Ingo Molnar <mingo@...e.hu>
> >  Date: Tue Apr 29 16:31:50 CEST 2008
> >
> >  Signed-off-by: Ingo Molnar <mingo@...e.hu>
> >  ---
> >   init/do_mounts.c |    8 ++++++--
> >   1 file changed, 6 insertions(+), 2 deletions(-)
> >
> >  Index: linux/init/do_mounts.c
> >  ===================================================================
> >  --- linux.orig/init/do_mounts.c
> >  +++ linux/init/do_mounts.c
> >  @@ -201,9 +201,13 @@ static int __init do_mount_root(char *na
> >         return 0;
> >   }
> >
> >  +#if PAGE_SIZE < PATH_MAX
> >  +# error increase the fs_names allocation size here
> >  +#endif
> >
> > +
> >   void __init mount_block_root(char *name, int flags)
> >   {
> >  -       char *fs_names = __getname();
> >  +       char *fs_names = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, 1);
> >
> >         char *p;
> >   #ifdef CONFIG_BLOCK
> >         char b[BDEVNAME_SIZE];
> >  @@ -251,7 +255,7 @@ retry:
> >
> >  #endif
> >         panic("VFS: Unable to mount root fs on %s", b);
> >   out:
> >  -       putname(fs_names);
> >  +       free_pages((unsigned long)fs_names, 1);
> >   }
> >
> >   #ifdef CONFIG_ROOT_NFS
> 
> It could have been a bug in early kmemcheck too. We don't check memory 
> allocated with the page allocator, only slab, so this shouldn't 
> trigger anything.

no, i tracked it down and the problem was some genuine weirdness in this 
code (and not in kmemcheck) but i forgot the details :-)

it was something rather disgusting, the boot parameter parsing stuff.

	Ingo
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