linux-kernel - Re: shmem_getpage_gfp VM_BUG

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Date:	Thu, 25 Oct 2012 07:14:11 -0400
From:	Dave Jones <davej@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

On Wed, Oct 24, 2012 at 09:36:27PM -0700, Hugh Dickins wrote:

 > > 1148                         error = shmem_add_to_page_cache(page, mapping, index,
 > > 1149                                                 gfp, swp_to_radix_entry(swap));
 > > 1150                         /* We already confirmed swap, and make no allocation */
 > > 1151                         VM_BUG_ON(error);
 > > 1152                 }
 > 
 > That's very surprising.  Easy enough to handle an error there, but
 > of course I made it a VM_BUG_ON because it violates my assumptions:
 > I rather need to understand how this can be, and I've no idea.
 > 
 > Clutching at straws, I expect this is entirely irrelevant, but:
 > there isn't a warning on line 1151 of mm/shmem.c in 3.7.0-rc2 nor
 > in current linux.git; rather, there's a VM_BUG_ON on line 1149.
 > 
 > So you've inserted a couple of lines for some reason (more useful
 > trinity behaviour, perhaps)? 

detritus from the recent mpol_to_str bug that I was chasing.
Shouldn't be relevant...

diff -durpN '--exclude-from=/home/davej/.exclude' src/git-trees/kernel/linux/mm>
--- src/git-trees/kernel/linux/mm/shmem.c       2012-10-12 10:01:46.613408580 ->
+++ linux-dj/mm/shmem.c 2012-10-15 12:31:32.979653309 -0400
@@ -885,13 +885,15 @@ redirty:
 static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
 {
        char buffer[64];
+       int ret;
 
        if (!mpol || mpol->mode == MPOL_DEFAULT)
                return;         /* show nothing */
 
-       mpol_to_str(buffer, sizeof(buffer), mpol, 1);
-
-       seq_printf(seq, ",mpol=%s", buffer);
+       memset(buffer, 0, sizeof(buffer));
+       ret = mpol_to_str(buffer, sizeof(buffer), mpol, 1);
+       if (ret > 0)
+               seq_printf(seq, ",mpol=%s", buffer);
 }


 > And have some config option I'm
 > unfamiliar with, that mutates a BUG_ON or VM_BUG_ON into a warning?

Yes, I do have this..

-#define VM_BUG_ON(cond) BUG_ON(cond)
+#define VM_BUG_ON(cond) WARN_ON(cond)

because I got tired of things not going over my usb serial port when I hit them
a while ago. BUG_ON is pretty unfriendly to bug finding.

	Dave

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